So far, this book has provided invaluable help in pointing out some of the more obscure and esoteric referenes and allusions in Ulysses. For example, Thornton tells you when certain parts refer to a popular Irish ballad or song that nobody except a hardcore Joycean scholar would know.
However, I do not feel that this guide alone is sufficient as there are a few obscure parts I have come across in the text which have no explanation in this book. Either Thornton is still figuring out what they mean himself or he felt they were self-explanatory but given Joyce's track record in prose, I doubt that the latter is the case.
I am reading this alongside the novel and in conjunction with A Reader's Guide to James Joyce by William York Tindall. Both books together are excellent help. I have heard the Bloomsday Guide is good too.