Materials on database indexing with examples
The wikipedia entry is informative. 

Database Systems: The Complete Book by Hector Garcia-Molina, Jeffrey D. Ullman, and Jennifer Widom  is probably good. Older versions of database books by Ullman have been accurate and reliable. I haven't seen the latest version, though. 



Actually, most of my experience with Ullman has been his compiler books, not database. Still, though, a reliable person. More than just a random author.