Going without context here, but "only book" may mean it's a doujinshi that focuses only on one particular character or pairing. Or one particular incarnation of a character, for instance the character as uke or as seme. Would that make sense for the doujinshi you're holding?

In the case of an "only event", it's actually noteworthy to mention that this is a convention that focuses on one fandom only, because a lot of conventions are multifandom. But the vast, vast majority of individual doujinshi are about one fandom only, so it would probably be superfluous to mention that fact on the cover too.

As for yorozu and crossovers: I'm not sure because I haven't actually read much yorozu dj, but I think yorozu is not used to indicate crossovers. There's クロスオーバー as a word for that, and I thiiiink there was also another word that I can't manage to remember right now. Will get back to you if I figure that out. You're not bugging me, I just don't know :D