I would recommend checking out 'Designing Great Beers' by Daniels. It is a bit outdated as far as information, but the methodology for creating recipes is good. I also went to the BJCP website (bjcl.org) and downloaded their style guidelines. From reading these and looking at clone recipes for the recommended commercial examples of a given style, I could get a feel for how much of a certain type of grain to add, how much hops and when, types of yeasts used for a given style. I also recommend 'Brewing Classic Styles' by Zainasheff and Palmer. Although I picked it up after designing my own recipes for many beer styles, it helped with confirmation that my recipes were pretty close to others that had been developed.
Now with having brewed 40+ all grain recipes of my own behind me, I use the style guides as a staring point and branch out creatively on my own with new recipes, just as a cook will.