I have two to recommend depending upon what you are looking for. First one is "the Adobe Photoshop Lightroom Book For Digital Photographers" by Scott Kelby (ISBN-13 978-0-321-49216-6) for about $40. This is a clear and concise step-by-step tutorial on how to do the most common edits using Lightroom as well as helping you learn the rudiments of organizing your photos. Lots of hints on how to do things but little depth about how things work or what all those other options and setting do.
The other book is "The Adobe Photoshop Lightroom 5 Book--The Complete Guide for Photographers" by Martin Evening (ISBN-13 978-0-321-93440-6) for about $60. This book is less about how to do a particular task and more about everything you ever wanted to know about Lightroom. There are some hint and tips and how-to's but this text goes into great detail about all of the setting and options available; what they do, how they work, how they can help you, how they can hurt you and even how to fix the hurt a badly chosen option has caused. It is not a book to read from cover to cover but is a complete reference.
If you can only afford one book, I would suggest Kelby's book. This will get you up and running faster and give you enough information to get your work done quickly and efficiently. It won't answer all of your questions but will still give you the basics as well as the possibilities. Some of the tutorials get into some pretty advanced stuff though and there is a companion website where you can even download sample images to work on as you go through the steps.