EDIT: I should clarify that this question is mainly historiographical. That is to say, the strengths and weaknesses of the primary sources aside (and I think there's actually a fair amount of Greek/Roman information on the matter), why aren't there many good (English) secondary sources on this matter?
I’m researching the contemporary enemies of the Roman army in the Republican period to provide comparative (or contrasting) material for the Roman army itself. Specifically, I am working on the matter of morale, with constructs such as ‘warrior culture’, ‘fighting spirit’ and ‘combat motivation’ being central to what I’m looking for. I have been having trouble in course of my research finding good secondary sources specifically on the matter of Celtic and/or Germanic warfare. The books I do tend to look into like “Ancient Germanic Warriors” (Speidel 2004) “Lords of Battle” (Allen 2007) do not seem to be well reviewed or particularly useful for academic work. Perhaps the best article I have read on this matter is the chapter by Wells, “The Germanic and Danubian Transfrontier Peoples” (Oxford Handbook of Warfare in the Ancient World, 2013, 589-600), but it is quite short.
There are valuable primary sources, such as the countless Roman accounts of battles with Celtic and Germanic enemies and the more analytical Germania of Tacitus (which I admit I haven’t read in detail) as well as the great detail given by Caesar in the Bellum Gallicum. Caesar has a long digression about Germans in particular which as far as I know is plagued by ethnographic clichés, but he knew a thing or two about the Gauls and so is the best primary source on the matter.
Later periods regarding the successors to these early Roman-era Celts and Germans, the Germanic successors in the Medieval period, appear to be better researched. I wonder if I should utilise some of the well-regarded works on this period to enlighten me (such as Bachrach’s “Early Carolingian Warfare” and Halsall’s “Warfare and Society in the Barbarian West 450-900), but I fear the peril here is that these cultures had developed so far from the period I am looking at (and the literary evidence remains so thin) that these later military cultures will tell me little about their predecessors.
I can’t access my university database at the moment, so that is hugely limiting the material I can access (though I recall there being some mid 20th century stuff floating around). But it seems bizarre to me that there is such a dearth of good scholarly work on the military aspect of Gallic and Germanic societies in the Republican and early Imperial Roman period. Can anyone enlighten me with some of the insights they have picked up on this matter? If what I say is true, why is it there is little good work on this matter (is it merely the lack of non-Roman evidence)? Can anyone direct me to some valuable reading? Thank you for your help in advance.
Have you read Adrian Goldworthy's The Roman Army at War? Given your flair, I'd assume yes, but he has several useful chapters on Gallic and Germanic style warfare.
Don't know if you have ever learned Latin (roman era) but a lot of the historians or writers could give you more information. You can find them in schoolbooks but everywhere else too. There aren't a lot of translations for some books so maybe that's why you don't know them yet.
Read about the recent studies of the Tollense battleground. Gallic and Germanic tribes are not known for their literary contributions, so most of what we have to rely on comes from Roman and Greek descriptions. Tollense is proof of the fact that the Gallic and Germanic world was way more complex than reported by the historians of the time