The
Harry Potter Lexicon
All things Harry Potter. Unfortunately, doesn't include all of OotP
yet, but has loads of stuff including lists of magical creatures,
items, and people.
The
Elements of Style by William Strunk
Asserting that one must first know the rules to break them, this classic
reference book is a must-have for any student and conscientious writer.
It concentrates attention on the rules of usage and principles of
composition most commonly violated.