The Birnbaum book makes a good pocket guide. I say that because at 5x7 and maybe a quarter-inch thick, it can probably fit in your pocket.
If you have read the Birnbaum's WDW books, it is pretty much the same. You won't find any honest criticisms of Disney. It is very colorful and has some decent detail to it.
But THE book on Disney cruises is the Passporter. They go into great detail on everything you need to know and give their honest opinions on things.
-steve