you totally need a guide for Ulysses
I really don't think it's comprehensible without buying some kind of "Ulysses for Dummies" companion volume. You need to understand Homer's Oddysey, Irish history and a lot of biographical facts from Joyce's life. It's worth the trouble, though, I think.
As for Finnegan's Wake, Levi don't do that. I think Jota read it.