My early Christmas present from my friends in CT was a copy of this book
Swedish Breads and Pastries: Amazon.ca: Jan Hedh: Books
It includes an excellent introduction to the ingredients commonly used when baking bread, as well as the terminology. The 2nd chapter is all about leavening agents--sourdough starters, Levain/natural starters, including raisin yeast and apple yeast. It explains how to move from the levain to the mother to the chef, with nice photos. And poolish, something of which I had never heard. It includes an good overview of dough temperatures, kneading, shaping loaves, and baking them. After one gets through the "primer," the recipes start. There are several different breads using levain, a number of traditional Swedish breads, and of course, there are pastry recipes (however, most of the recipes center around bread). If you have a bread baker for whom you'd like a gift under $30, this book would, I think, be a much appreciated gift. Once I get Christmas and New Year's out of the way and have a bit more time, I'm diving into trying some of the recipes using natural starters.