Flour. What I love about Flour is that it feels like being at a much better version of a bake sale. The cookies, bars, cakes, and cupcakes are all better versions of things you ate growing up: homemade oreos, granola bars, cupcakes, etc.
So, it didn't take me long to buy Joanne Chang's cookbook, Flour: Spectacular Recipes from Boston's Flour Bakery + Cafe. I highly recommend this book. It is clearly written, and packed with photographs. The introduction sections are worth the purchase alone -- Chang offers a simple, clear primer on baking, and my baking has improved since reading this! I think she convinced that it is worth being a little less cavalier about measuring, etc, when baking (which was always one of my bad habits).