Michael Ruhlman's Ratio is an excellent book detailing not just some recipes, but why recipes have what they have (and in the quantities they do). I found it's great to help free your mind from following recipes blindly and move to making your own (or improvising more).
Plus, some of the recipes in the book are quite good on their own. They also tend to be either simple enough to easily modify or have the simplified ratio detailed so that you know what's optional and what's mandatory.
His blog is also quite good a highlighting certain ratios and encouraging me to try new dishes.