Relational Database Index Design and the Optimizers (Wiley, July 2005) is an excellent book on how indexes work in general in a relational database. The authors focused on principles and not any particular database vendor.
The book focuses on two things; first, the database optimizer of the relational system has to decide how to find the required information in more efficient way, and secondly how the tables and indexes are scanned. Simply put, the principles shown in this book allows you to put yourself in the optimizers place – Also, shows how one can use this knowledge to quantify the work in terms of CPU and elapsed time.
The authors have done a fantastic job of describing a solid methodology that helps you diagnose performance problem, do a proactive and reactive tuning. Again, it is an excellent book, if you accompany with the performance tuning guide of your database.
You can take advantage of google books to preview the content. As you read, you will realize that this book does not give you any rules of thumb; it only offers the principles behind a good index design and will also shift the way you look at indexes.