I don't know whether or not others will find it useful, but I thought I'd put together a list of references that I've found useful (or even not so useful) in putting together my Dark Ages stuff.
I've deliberately left game rulebooks and supplements out of the list. These are specific to whatever setup you use, and having a pile of rulebooks for something you don't play probably isn't useful.
Anyway, on with the list.
Wargames Research Group
Armies of the Dark Ages, Ian Heath, WRG
Probably as near an indispensible reference as there is, in my opinion. If you really could only have one book, this would be a compelling choice. Follows the usual WRG format of an overview of the armies of the period, potted battle histories, plus a huge number of black and white line illustrations with informative captions.
Armies and Enemies of Imperial Rome, Phil Barker, WRG
Arguably a bit too early, but contains a good deal of stuff that is both interesting and useful, especially for an early Dark Ages player.
Availability of WRG books seems to be a little bit hit and miss, but they are well worth investing time hunting down on Ebay.
The various Osprey series probably needs no introduction. I bought my first MAA book about 27 years ago and have carried on ever since. Don't expect tremendous depth from a book with only 48 pages, and you are usually not too badly disappointed. The colour plates of course can be a great source of inspiration. Particular volumes I've found useful include:
MAA154 Arthur and the Anglo-Saxon Wars, David Nicolle and Angus McBride.
Becoming slightly dated as the archaeology and its interpretation has moved on, but still a solid source, especially the late Angus McBride's plates.
MAA247 The Roman Army from Hadrian to Constantine Michael Simkins and Ronald Embleton
Useful as a source for Romano-British armies.
Warrior 17 Germanic Warrior AD 236–568 Simon MacDowell and Angus McBride
A very broad subject indeed, but MacDowell provides a useful jumping-off point for more detailed research. The illustrations have proved very useful indeed.
Warrior 50 Pictish Warrior AD 297–84 Paul Wagner, Angus Konstam and Wayne Reynolds
Very little is written about the Picts, so the choice of references isn't large. The illustrations are useful, but not in the same league as McBride's work. Whilst I have unashamedly used this book on many occassions, given the very sketchy source materials the authors have had to work from, I have lingering reservations about how truely authoritative it can be.
Warrior 9 Late Roman Infantryman AD 236–565 Simon MacDowell and Gerry Embleton and
Warrior 15 Late Roman Cavalryman AD 236–565 Simon MacDowell and Christa Hook
The influence of late Roman tactics and equipment, especially armour, on Dark Ages warfare, is profound. For this reason alone both of these books are worth having. MacDowell's written work is very good, clear and authoritative. Unfortunately the illustrations do not equal it, in my opinion. I do not especially like the style of either illustrator, and whilst the colour plates are useful, I have not found them as easy to work from (for example when sculpting helmets) as similar offerings from McBride or even other work by the same people.
Warrior 5 Anglo-Saxon Thegn AD 449–1066 Mark Harrison and Gerry Embleton
Very useful. Harrison's written work is a good starting point, and Embleton's illustrations are useful, certainly better than his work on the Roman Infantry. I think this is currently out of print, but worth seeking out.
Fortress 92 Strongholds of the Picts, Angus Konstam and Peter Dennis
Interesting and useful, with illustrations and text that are very handy for applying to the general principles of how Pictish fortresses might have been constructed. The black and white illustrations are in many cases too small to be helpful and are merely tantalising, but the decent bibliography allows you to get to the originals.
Fortress 80 British Forts in the Age of Arthur, Angus Konstam and Peter Dennis
Konstam and Dennis's works are useful if sketchy sources, and could use a more brutal hand with the editing in a number of places. However, the descriptions and the colour plates are fantastic as source material and inspiration for building model fortifications.
Fortress 56 Rome’s Saxon Shore, Nic Fields and Donato Spedaliere
Text OK, generally informative, illustrations not too great at all.
I make no pretense of having any academic qualifications in this field (I'm a chemist), but it is gratifying to find the volume of academic work available that there is. Some disciplines appear almost to hide their work from the lay reader. None of the sources mentioned here are in very heavy going - anyone who can handle the work of Phil Barker will be all right.
The Grammar of Anglo-Saxon Ornament, Professor Rosemary Crump
Part of the Corpus of Anglo-Saxon Stone Sculpture, and available on the web here. An essential and very accessible guide to Anglo-Saxon stonework, invaluable in producing wargame buildings.
The Hebridean Iron Age: Twenty Years’ Research, Professor Dennis Harding
If you want to build a Pictish structure, read this paper. A fantastic piece of work, available here.
Symbols of protection: the significance of animal-ornamented shields in Early Anglo-Saxon England, Tania Dickinson.
A great guide to early medieval shields. Less detailed than Dickinson and Härke's co-authored work, but easier to find.
The Anglo-Saxon weapon burial rite: an interdisciplinary analysis, Heinrich Härke
A guide to who was buried with what, and an attempted to rationalise why. If you want some evidence for why most men had spears only, this is it.
I'll update this with new references and new sections as and when. I hope it's useful to someone.