There's quite a few different books that I would recommend reading, although it depends on what your particular interest is in Alton Towers whether you would find them interesting.
Michael Fisher has published a few fantastic books over the years which give a sort of timeline of the development of Alton Towers from a hunting lodge to the grounds and buildings that we see today, as well as some parts relating to the development of the theme park. The first being Alton Towers: A Gothic Wonderland originally published in 1999, it can now be purchased for 1p on Amazon apparently. A revised version incorporating a lot of new information and also bits about the ongoing restoration projects was released in 2009, this time called Alton Towers: Past and Present. I would certainly recommend purchasing both if you can. In Towers Trading you can sometimes purchase pamphlets that contain abridged versions of these books.
Another excellent book is Discovering Alton Towers by Malcolm McIntosh, published in 1988. Whilst not being a book that you necessarily read, it's more of a guide book, it contains some excellent photos and information from the early days of the theme park. It can sometimes be a bit tricky to get hold of, but there are a few on Amazon at present at reasonable prices.
Alton Towers themselves published their own guidebooks more or less year after year during the late 1950's and early 1960's, then more sporadically afterwards. These tend to pop on eBay every now and again, they're well worth a read as they contain some good information and more excellent photos. We have a few of these scanned within the TT archives, for your viewing pleasure!
Last but by no means least is John Wardley's autobiography, Creating my own Nemesis. Whilst not focussing on Alton Towers for the whole time, it does contain some amazing insight into the creation of some of its signature rides.
Enjoy your reading!