SQLite is a small, fast, embeddable database. What makes it popular is the combination of the database engine and interface into a single library as well as the ability to store all the data in a single file. Its functionality lies between MySQL and PostgreSQL, however it is faster than both databases.
In SQLite, author Chris Newman provides a thorough, practical guide to using, administering and programming this up-and-coming database. If you want to learn about SQLite or about its use in conjunction with PHP this is the book for you.