I'm writing an undergraduate paper on what state England was in militarily before the major Viking attacks at the end of the tenth century and beginning of the eleventh (i.e. during Ethelred II's reign), hoping to explain why the system created at the end of the ninth century and beginning of the tenth by Alfred and Edward failed to protect England a century later. What I've found is that there are many primary and secondary sources on the creation of the Burghal System etc. (ASC, the Burghal Hidage, etc.), and quite a lot concentrating on Ethelred's reign (ASC again, Keynes' and Roach's work to name a few). Some of the sources I've seen have even gone so far as to basically blame the peaceful period of Edgar's reign for the decay of the system (Abels is one, in Lordship and Military Obligation in Anglo-Saxon England).
The problem I seem to be facing is that most of the sources I can find for Edgar's reign itself choose not to focus on any of the military aspects, concentrating on monastic reform and his coronation instead. While those are interesting for different reasons (and the coronation does sort of tie into the whole military thing with it kind of maybe showing submission of local kings to him, I do understand that), they're not the things I'm looking for at the moment. Does anybody have any ideas for more sources (primary are good, but I'm especially dying for secondary reading, preferably accessible online but I'll take what I can get) I can look at?
First off, it sounds like you've already accomplished some good secondary research. Abels is top notch, as are Keynes and Roach. Depending on the length of your paper, just dealing with those three might be a really good historiographic basis. I'd recommend writing a paragraph (250 words) summarizing each of them: What is the main argument (one sentence)? What are the main supporting arguments and bits of evidence? Why does the author think this is important? I do this for all major works (and even important articles) in my field. It sounds like you've already done this—at least in your head—for Abels. Then think about where these authors disagree with each other, and where you disagree or think more can be said. That becomes the main argument of the paper and maybe an outline for the three or four supporting subsections of the paper.
That's how I sometimes shape my arguments. The other way is jumping into primary sources, which also seems to be something you've done already as well. With the ASC, be careful in how you're using the different manuscripts of the Chronicle, which all tell a different story. I've always found the introduction to Swanton's edition to be approachable and easy to use.
So where to go from here? For secondary sources, see if your library has access to the International Medieval Bibliography. I often use this and, to a lesser extent, the Bibliographie de civilisation médiéval. Both are online databases that tell you what's been published on a topic, and most large university libraries have them listed in their online catalogs. You might also want to search for Ethelred or Æthelred in some of the key journals, which might include Early Medieval Europe, Speculum, the Haskins Society Journal, the Anglo-Saxon England journal, and Anglo-Norman Studies. After searching these things, I'd also recommend (1) checking to see if your library has a reference librarian that can help you identify further materials, and (2) talking with your professor. Sometimes professors have certain interests which make them tick, and he or she might think that any paper on your topic might need to engage with a particular work or problem.
For primary sources, there's a number of good options. I'd recommend checking the Viking-Age Reader and Whitelock's English Historical Documents, 500-1042, to see if these have anything you haven't seen yet. I'd also recommend looking through the Prosopography of Anglo-Saxon England (PASE) website. It can be a bit clunky to get used to, but it's a phenomenal resource. Find Æthelred 32. I'd recommend clicking 'Order by Source' on the righthand window that pops up, and then scrolling down. Each of those 'Factoids by Source' is a different primary source. (Here's a stable link.) Most of the source titles start with an S, which indicates that they are charters indexed by Sawyer. There's also an amazing resource for these—Electronic Sawyer. You can find all the diplomas from Æthelred II here. Most of these will be in Latin, but some include a translation, such as this one. You'll see that one of the clauses indicates that the recipient of this land owes military service, bridgework, and fortress-work. The key reference for understanding these common burdens is: Nicholas Brooks, “The Development of Military Obligations in Eighth- and Ninth-Century England,” in England before the Conquest: Studies in Primary Sources Presented to Dorothy Whitelock, ed. Peter Clemoes and Kathleen Hughes (Cambridge: Cambridge University Press, 1971), 69–84. I don't believe this is anywhere online, but it can certainly be requested from your library, and they'll probably email you a pdf in a couple days.
I hope that helps. Most of my work is pre-900, so I'm not up to date on the literature about Æthelred, but these are the different places I'd go for information. Best of luck!