Same here! It seems like a fascinating system, especially in the federated way in which it's being used by various Japanese transit agencies and issuers. Compared to MIFARE, it was definitely ahead of its time.

Unfortunately, most things I could find are in Japanese, as expected; I suspect that the really interesting parts aren't public, as usual in this industry (there's still a lot of belief in security by obscurity, even if the systems actually don't need it).

Singapore's CEPAS seems very similar conceptually to Felica (at least in application, in that there's multiple issuers of stored-value cards with interoperability), and the specifications for that seem to be available for purchase, but I'm not curious enough to bite that bullet yet :)