I am not a regular umpire but enjoy lurking on the board to learn more about the rules. I thought this would be an interesting book to acquire so I went looking for it as well. While the link posted earlier does direct you to a place where it could be ordered I also found this link:
http://macroweb.com/ibrules/
On this page you can generate a random quiz choosing the number of questions that you want, take the quiz, have it graded and then for each answer have an explanation of why the call was made including the rules reference.
I have taken a few quizzes at this point and it is nice to be able to "Blow The Call" without causing problems with Players/Managers/Fans. I also think that I will better retain the correct answer this way. I don't know if it would be of interest to anyone but thought that I would share.