Update (July 13, 2014): I’ve been informed that I should be looking at hierarchical topic models (see Blei’s papers here and here). Thanks to Reddit users /u/GratefulTony and /u/EdwardRaff for bringing this to my attention. However, Redditor /u/NOTWorthless says HDPs do not provide a ‘posterior on the correct number of topics in any meaningful sense’. I’ll do more research and do a follow-up post.