The following are other 'Dark Age' and related texts that might be of interest1. They are listed roughly in the order they are thought to have originally been written. It is far from being a complete list, so you might also be interested in the following other sites:

In some cases the texts below are relevant to the Historia, in some cases they are just generally interesting, while in others the links are just there because they are hard to find on the net.

Name Date Notes
Pytheas of Massilia's Peritou Okeanou (On The Ocean) c.320 BCE. Now lost book by early mediterranian traveller (Biography) who may have travelled up Britain and possibly as far as Iceland. Fragments of the work have been quoted by later authors. British sections can be found here.
Constantius of Lyon's de Vita Germani (The Book/Life of Germanus) Before 494 CE (c.475-80 CE) An different life of Germanus (the "Liber Beati Germani") is noted as used in the Historia, though the author seems to have conflated it with details of St.Garmon, a local Welsh Saint, and done some generous interpolation (see Dumville*, though it isn't entirely clear which Manuscript the Liber is mentioned in). You can find a detailed discussion, and extracts of this later version here and here.
Early Welsh texts c. 9th C Juvencus Englynion: Early examples of Welsh verse. You can find the Latin and an English translation here. The Computus Fragment: A chronological calculation sheet. A slight commentary can be found here. The Surexit Memorandum: A legal report. You can find a details and a translation here.
Gildas' De excidio Britann(i)ae liber querulus ("the Fall/Ruin/Capture of Britain, a whining book") Written c.545 (Biography; Biography) CE*

The venomous De excidio Britann(i)ae is regarded as one of the more reliable accounts of the age. It is a gloriously tirade against the sinful uselessness of the British. It usually divided into three parts : The Preface, The History and The Epistle. You can find the English translation from Giles* of the first two, with his notes, here, or the complete thing, without notes here.

Y Gododdin / Canu Aneirin (The Book of Aneirin : Cardiff MS 2.81) c.13th C (but possibly 6th C originally) Poem from the 'Book of Aneirin' detailing the destruction of a British war force sent from the Scotish borderlands to remove invaders from Yorkshire. You can find a translation of Y Gododdin here. John Koch* has attempted a reconstruction of the possible 'original' poem. Griffen* has an accessible description of the evidence for an early date.
The Tribal Hidage c. mid-7th to mid-9th C A summary of Saxon land holdings, chiefly concentrating on Mercia (Info). Various versions can be found at Tony Jebson's site (here).
The Ravenna Geography c.7th C A list of Roman constructions and towns. You can find a copy of the text for Britain, plus a translation, as well as the British sections of older geographical works such as Claudius Ptolemaeus' ("Ptolemy": biography) Geographike hyphegesis ("Guide to Geography": 2nd C) and the Antonine Itinerary at Kevan White's Roman Britain site.
Canu Heledd 9th C (but possibly 7th C originally) A set of poems describing the fall of the British Cynwydion dynasty of Midland England. The Welsh, and an English translation, can be found on Keith Matthews' site.
Englynion Y Beddau ('The Stanzas Of The Graves') (possibly c.9th C, but see "Black Book of Carmarthen", below) A poem of seventy-three stanzas, themed around the graves of heros. You can find a growing list of translations here.
Historia ecclesiastica gentis Anglorum 731 CE Bede's "Ecclesiastical History of the English People" gives a narrow but well-held history of the English people, and the British as far as they got in the way. A growing biography of Bede with links to his works can be found here.
Anglo-Saxon Chronicles 891 CE+ Cover-all name for a set of chronological manuscripts describing, generally, the major events in the Anglo-Saxon settlement in England (Info). You can find the an English translation here.
Armes Prydein / Preiddeu Annwn ("The Spoils of Annwn") / Kat Godeu ("The Battle of the Trees") c.930 CE / possibly 8th C / younger than 6th C : but all found in the early 14th C "Book of Taliesin" (see below). The Armes Prydein is a prophetic poem, while Preiddeu Annwn ("The Spoils of Annwn") is about an attempt by Arthur to seize a cauldron from the underworld. Kat Godeu is about a battle fought by moving trees. Griffen* has an accessible description of the evidence for an early date for Armes Prydein.
Welsh / Anglian dynastic genealogies / Bonedd Gwyr y Gogledd (Pedigrees of the Men of the North) 10th C* onwards These are a series of genealogies, including one group attached to Manuscript A*. You can find them at Keith Matthews' site here. You can also find a detailed discussion of them in Dumville*.
Annales Cambriae (The Annals of Wales) Version A c.1100 CE but could be down to 977. These are available from three early sources, the oldest of which is attached with the Historia in Manuscript A. You can find latin versions of two of them on Keith Matthews' site here. You can find an English translation in Morris* and online here.
Leges Howelda The Laws of Hywel Dda 10th - 13th C or earlier A glorious compilation of early Welsh laws2. Hywel died in 950 CE, but the laws may be earlier or later. Earliest manuscript is Mid 13th C For these, and other details, plus scans see here. A translation and notes can be found in Richards*
King Lists Various Various lists of Kings and their reigns, drawn up for the purpose of chronology. Keith Matthews has compiled a variety of manuscripts containing these, including the 'Moore Memoranda' (a series of annotations on the oldest copy of Bede from 737*) in one site. Again, a discussion can be found in Dumville*.
Liber Landavensis c.1132 CE* "The Book of Llandaff/Llan Dâv" - a compendium* of works including Saints lives, charters and a gospel of St. Matthew. Includes a large number of boundary descriptions, almost certainly older than the period of compilation*. A well respected edition is Evans*, which gives the boundaries in translation.
Caradoc of Llancarfan's The Life of Gildas c.1130 to 1150 CE A somewhat speculative life of Gildas. Its mention of the fatal animosity between Arthur and his brother is sometimes used as justification for Gildas' failure to mention Arthur. You can find an English translation on Mary Jones' site.
Geoffrey of Monmouth's (biography) Historia Regum Britanniae (History of the Kings of Britain). c.1129 to 1151 CE A popular history of Britain, the main sections of which are the foundation of the island by Brutus, the rule of Constantine, and the rule of Arthur. Now (and indeed, then3) regarded as confused and largely fantastic. Geoffrey's claim that it is a rewrite of an older British book has caused some difficulties, but it seems that the Historia Brittonum was used in part.
Geoffrey of Monmouth's (biography) Vita Merlini (The Life of Merlin). c.1149 to 1151 CE Geoffrey's follow up to his Historia Regum Britanniae (History of the Kings of Britain). You can find the latin and an English translation here.
Welsh Bruts Various A series of chronicles from Wales - most are worked around Geoffrey of Monmouth's Historia Regum Britanniae. For example, the sorry history of the Jesus College MS LXI Brut (Sometimes known as Brut Tysilio : The Chronicle of Tysilio) can be found in Skene* Chapter four. It is broadly considered to be a reworking of Geoffrey of Monmouth's Historia Regum Britanniae (though some consider to be its original), while Brut y Tywysogyon is a continuation of it (Pen. 20 version dates from late 14th C but from an older Latin original, probably based on the Annales Cambriae, now lost*)
Llyfr Du Caerfyrddin (Black Book of Carmarthen: NLW Peniarth MS 1) c.1250 CE Collections of Welsh poems and some of Trioedd Ynys Prydein ("The Welsh Triads", also found in 13th C NLW Peniarth MS 16, part iv, ff.50-4v, The Red Book of Hergest, and elsewhere: you can find a collation here) including a works number on Dark Age heros, including Arthur. Includes the Englynion y Beddau ("The Stanzas of the Graves"), Pa ?r yw'r Porthor ("What man is the gatekeeper") (in which Arthur lists his men) and a number of poems which may or may not be about 'Merlin'. You can find an online copy here, and scans and a history here.
The Hendregadredd Manuscript c.1282 - 1330 CE Collection of Welsh poems. You can find out more and see the manuscript here.
Llyfr Taliesin (The Book of Taliesin: NLW Peniarth MS 2) early 14th C Collection of Welsh poems of a variety of ages attributed to Taliesin. Of these, the most notable are probably Armes Prydein (see above), and Preiddeu Annwn ("The Spoils of Annwn" : in which Arthur attempts to seize a cauldron from the underworld). Meirion Pennar's extraction from these of the most likely original poems of Taliesin can be purchased from Llanerch.
Llyfr Gwyn Rhydderch (The White Book of Rhydderch: Peniarth MS 4/5)/ Llyfr Coch Hergest (The Red Book of Hergest: Jesus College MS 111, now Bodleian Library) c.1325 / c.1400 CE Collections of Welsh folklore, poems and stories, including those (since Guest*) known as the Mabinogion (the White Book is fragmentary in places). You can find a online version of part of the Red Book here. You can find out more, and view scans of the White Book here. The Red Book includes some of Trioedd Ynys Prydein ("The Welsh Triads", also found in 13th C NLW Peniarth MS 16, part iv, ff.50-4v, the Black Book of Carmarthen, and elsewhere: you can find a collation here)
Red Book of Talgarth pos. c. 1382 to 1410 CE Contains a poem themed around sayings, put in the mouths of heros. A selection in English can be found here. N.B. Published first in the dubious Y Myvyrian Archaiology (see below).
NLW MS Peniarth 51 c.1460 CE* Includes the earliest copy of Tri Thlws ar Ddeg Ynys Brydain ("The Thirteen Treasures of Britain": known in this copy and others as "of the North", there were up to 15 in collections of 13 in different manuscripts)*. You can find a list here.
Y Myvyrian Archaiology
"The Iolo MS."
1801 to 1807 The Myvyrian is a collection of Welsh classics which unfortunately includes a large number of forgeries. The editor suspected of including the forgeries "Iolo Morgannwg" (1747-1826) collated a followup collection of manuscripts which was published posthumously as "The Iolo MS.", and which were even more contaminated. For details see Mary Jones' site and Skene* Chapter four. Most biographies of "Iolo" are in Welsh, but a short English biography is Morgan*.