Hi, been reading a little bit about the Crusades in the Baltic and I'd really love to learn more, could anyone recommend any good sources about the military campaigns specifically or the more general history/society/culture of the Baltic at the time? I'm particularly fascinated by the existence of 'officially' pagan states in Europe so late into the Medieval period. Thanks in advance for the help :)
The amount of recent publications of this topic is certainly considerable, and you can easily recognize the advance (flood?) of literatures just by comparing the bibliographies attached to the following two essays (2001 and 2009 respestively) by Allan V. Murray.
'Culture(s) Clash' has indeed a kind of key word for the topic since the end of 1990s, since there was once CCC: Culture Clash or Compromise research project focusing on the political-religious-social-cultural transformation of the Baltic Frontiers from ca. 1100 to ca. 1400, whose project leader had been Nils Blomqvist. If you are really interested in this field of study, I'd recommend you to look for the reports of this research project, titled as 'CCC Papers'. And the work of Henry of Livonia has been extensively studied especially Marek Tamm.
Note that German is also probably a prerequisite for researching this topic further. The following book list comprises of mainly general works in English only, but some of them had been originally in German (or in Danish). If you can understand the original language, I'd recommend to look for original (since they are much less expensive than English translations except for North's German original).
While [Rowell 1994] is a bit old, it should be still very rare and complete accounts of Lithuanian history during the last period before their ruler's conversion. If you find it too expensive or too hard to find in your university's library, instead you can consult on her article in New Cambridge History, vi. R. Fletcher's Conversion of Europe (1997) 's section of Lithuania is also mainly based on her narrative, I suppose.
[Added]: If you've just done with Christiansen's Northern Crusade (1980; 1998) and a few general articles, I'd recommend to start with North or Rowell's article, 'Baltic Europe,' in NCM vi.