First, there is an unannotated list of books on cellular automata [here][1].
Second, if you are going to get just one book, then I think it has to be Wolfram's *A New Kind of Science*, which, despite its flaws, is the source of so much of the research in cellular automata
that it must be confronted first.  [I see I am concuring with Kevin O'Bryant's just-posted recommendation.] You might read [the review][2] by Lawrence Gray in the *Notices of the American Mathematical Society* (February 2003) to make
yourself aware of the controversies surrounding this book.  If you can overlook its flaws, it is a remarkable book, and quite fun to work through.


  [1]: http://uncomp.uwe.ac.uk/genaro/Cellular_Automata_Repository/Books.html
  [2]: http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.113.7950