[RPatrino]

Don't forget, the true measure of an umpire is not how many rules he knows, its how he applies the rules he does know in the heat of a game. There are many arcane, hardly ever used rules all codes, and it pays to read all of them. The facts are that some you will never have to use at all, others you will use in every game. There are a few that always will bite you in the butt if you are the least bit unsure, those being in my opinion ( others will hopefully add more to the list):
- Dead ball awards
- Pitching regulations
- Batting out of order
- Substitutions
- Obstruction/Interference
- Force Play Slide Rules
- DH rules

Again, there is no substitute for learning umpiring on the field. Memorization of rules with no link to real life situations on the field will not help you. Don't be a rule book umpire, be an umpire with solid mechanics and field presence who has a solid grasp of the rules and how to apply them.