There are literally hundreds of different cruise guide books available so you need to decide what you want to get out of a particular book.
If you are new to cruising then one of the main annual complete guide to cruising books like the "Berlitz Complete Guide to Cruising & Cruise Ships" from Berlitz or "Stern's Guide to the Cruise Vacation" from Stern would make sense. These will tell you everything you need to know about choosing a cruise and in particular the type of ship or cruise line to go with.
If you are planning on visiting a particular region or port, then you are probably better off going for one of the destination guides which will give you plenty of specific information about the countries and places of interest.
You will even find books on working on the cruise ships, how to sell cruises and inside information on the cruise industry itself.
There are also plenty of historical books about the classic cruise liners like the QE2 if you are interested more in the ships themselves.
