Yeast! It's wonderful, and now it's available in dry form, liquid, and paperback.
The new book version by Chris White (of White Labs) and Jamil Zainasheff (of homebrewing) is out now. I recently gave it a read and was very pleased. As the most important, yet often ignored, ingredient in beer, I'm always glad to read/talk/blog about yeast. Controlling yeast and fermentation is also probably one of the most challenging and misunderstood aspects of brewing.
Enter Yeast The Practical Guide to Beer Fermentation. As the title indicates, they cover everything yeast-related in the book. It starts with some background information on yeast and biology, but the bulk of the book focuses on conducting and controlling fermentation to get what you want from your yeast. There are also sections on troubleshooting yeast off-flavors as well as information on starting your own yeast lab. This book probably isn't for the beginning homebrewer, as it does get technical at points and the focus is very specific. The flipside is that they also cover a lot of very basic info, so anyone who might be interested in this book will probably be skipping over several sections in the beginning. Sections on yeast handling are probably more aimed at micro brewers and people in the professional beer industry (or some of us who get very excited about yeast).
For those homebrewers who are more experienced or are looking for something read beyond How to Brew, though, Yeast is definitely worth the read. The depth of information on yeast starters, yeast biology, and yeast-handling exceeded my expectations. For brewers who are looking to take the next step in improving their beer, looking at yeast is a great place to start. Yeast The Practical Guide to Beer Fermentation sits on my shelf somewhere in between those classic beginning-brewing authors (like Palmer and Papazian) and the more difficult to understand ones (like De Clerck and Fix). You can buy it here now.