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 :)
It's not really on the technical side of thigns, but the unification of IC cards across transportation companies has a good write up [0] that has a bunch of fun details. My favorite thing is how the people working on the project just had a bunch of card readers from various transport companies all in one room.
One small caveat: As much as I appreciate his writing, I'd take some of the technical explanations with a grain of salt – the approach is definitely more that of an (extremely knowledgeable and experienced!) outsider looking at the system and coming up with hypotheses for its workings than that of an authoritative source with hands-on experience working on the system. It's sometimes not that easy to figure out what's hypothesis and what's "confirmed" knowledge as a result.
That said, much of what I've learned about Japanese transit payment systems (without ever having visited) was via that blog. It's amazing!
Somebody from the tiny intersection of people apparently having hands-on experience with Felica and writing about it in English is this pseudonymous Reddit user (often also quoted on the blog): https://www.reddit.com/user/FelicaDude/
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 :)