For the various elements of the Northern Crusades, material seems a bit thin on the ground -- there's Christiansen's book, and then a bunch of maddeningly tantalizing, out-of-print stuff (William Urban's books in particular).
For the Old Prussians, stuff about "the other Prussia" tends to get in the way of my searching...
Samogitia, on the other hand, just doesn't seem to feature much at all -- at least, in popular works. Wider Lithuanian history has the opposite problem: there's material there, but I'm mainly interested in Lithuanians before ~1500, and I'm not sure what title will focus best on the Lithuania of the Crusade(s) and those centuries between prehistory and the creation of the Commonwealth.
(Finding English translations of primary sources is something I've almost given up on -- there's Henry's Livonian Chronicle, but the only English version of Balthasar's, or of the Rhyming Chronicle, seems to be in an again out-of-print compilation. So close, and yet so far...)
Thank you very much for any reading (or browsing) recommendations on any of these topics!
Unfortunately, most of the academic literature on the Baltic Crusades is in German. But there are a few other English secondary sources that you should be able to get hold of. William Urban and Eric Christiansen are among the few scholars that have published on the Baltic Crusades in English in a non-academic press, and so their books are the easiest and cheapest to get hold of. I'm a bit doubtful of the current value of Christiansen's book on the Northern Crusades. It's about 35 years old now and is rarely cited in recent literature, at least that relating to the Teutonic Knights. A more current scholar would be Alan Murray:
Alan Murray (ed.), Crusade and Conversion on the Baltic Frontier 1150–1500 (2001).
Alan Murray (ed.), The Clash of Cultures on the Medieval Baltic Frontier (2009).
A number of his recent articles are free online, the ones marked as open access: https://www.leeds.ac.uk/arts/profile/20046/875/alan_murray/publications
More general works on crusading and the military orders also touch upon the Baltic Crusades:
Norman Housley, The Later Crusades (1992).
Helen Nicholson, Templars, Hospitallers and Teutonic Knights: Images of the Military Orders, 1128-1291 (1993).
Desmond Seward, The Monks of War (1972).
For primary sources, things are quite limited, particularly in English translation. Also, these are often from academic presses and so are very expensive. Henry of Livonia's chronicle and the Livonian Rhyming Chronicle that you mention are the two main primary sources for the Baltic Crusades, at least from the perspective of the Teutonic Knights and their associates. A selection of extracts relating to the Baltic Crusades in other primary sources has been translated and put online:
http://sourcebooks.fordham.edu/halsall/source/12-13Cbalticcrusade.asp http://deremilitari.org/primary-sources/ (they're not categorised separately in this link, so you'll have to search through a bit)
There are a few other published sources that touch upon the crusades. Jan Dlugosz's chronicle is largely focused on Polish history, but often ties in with the Baltic Crusades and the Teutonic Order. Another interesting source, and one freely available on archive.org, are the treasurer accounts of Henry Bolingbroke (later Henry IV of England) during his crusade expeditions to the Baltic and the Levant in the 1390s. This edition also contains extracts from various chronicles and other sources referring to these expeditions. However, the text is untranslated.
Lucy Toulman Smith (ed.), Expeditions to the Holy Land and Prussia made by Henry Earl of Derby (afterwards king Henry IV) in the years 1390-1 and 1392-3. Being the accounts kept by his treasurer for two years. (1894). https://archive.org/details/expeditionspruss00camduoft
Jane Allan (ed.), The Annals of Jan Długosz: an English abridgement (1997).