You want to create some tunes but don't know where to start? Well, you don't really need any book to make music if you have a good ear but it certainly doesn't hurt to know one or two things about music theory and composition.
So, let me recommend you a couple of books.
First one (music theory) is:
Music Theory: A Practical Guide for All Musicians (Music Instruction) by Barrett Tagliarino.
Second one (music composition) is:
The Complete Idiot's Guide to Music Composition by Michael Miller.
There are other books out there about music theory and music composition, but those two have my seal of approval.
You really need to know about music theory because it actually makes things easier for you. Knowing the scales and intervals will help you tremendously when you are faced with the familiar piano roll and a bunch of empty patterns. Chords are a bit intimidating at first but they actually become rather obvious when you know about intervals.
Reading about music composition is not that important if you have a lot of experience writing tunes since it's kinda something you may figure out as you go along. Still, there are some good ideas to pick up and it may save you some time in the long run. Knowing about stable and unstable tones in a scale is something everybody should know about, for example.