The Introduction to James Joyce provides a useful commentary and is helpful for gleaning a wholistic view of his works. It is highly recommendable for those attempting to read one of his works without the benefit of having read all of his works, as it will give them an overview of plots, themes, characters, and historical background to Joyce's works that the reader may not have included in their own studies. I would also recommend this introduction to those who are doing extensive studies in Joyce's work since it provides useful historical contexting, as well as some common themes. That way, the reader is not forced to resort to giant tomes on Irish history or James Joyce's biography. Overall this is a good resource no matter what your familiarity with Joyce.
"Every life is in many days, day after day. We walk through ourselves, meeting robbers, ghosts, giants, old men, young men, wives, widows, brothers-in-love. But always meeting ourselves.”
“While you have a thing it can be taken from you... but when you give it, you have given it. No robber can take it from you. It is yours then, forever, when you have given it. It will be yours always. That is to give."