If you have enjoyed working with Mathematica, you will almost certainly enjoy The Mathematica Book. It is available on the web, through Stat/Math Center Sales, at your local bookstore, and in the online help.
In addition, be sure to wander through the Wolfram Research Documentation Center, where you will find reference works on any topic imaginable.
In addition, your local bookstore or library probably has several books about Mathematica or one of its applications. WRI keeps a list of current books on their website.