For recipe design, I also recommend Designing Great Beers...very in depth.
By far the easiest way to design recipes, extract or AG, is to get some software like ProMash. It will help you get the color, bitterness and gravity right within the style guidelines. There are still a lot of variables, but if you nail those 3 things, you'll usually make drinkable beer. Cheers!
Oh don't give me none more of that Old Janx Spirit
No, don't you give me none more of that Old Janx Spirit
For my head will fly, my tongue will lie, my eyes will fry and I may die
Won't you pour me one more of that sinful Old Janx Spirit