There's Money Where Your Mouth Is by Elaine A. Clark is as subtitled: A Complete Insider's Guide to Earning Income and Building a Career in Voice-Overs. In only 23 chapters, Clark covers everything a beginner to voice-over could possibly want or need to know, from simply what voice-over is, to recording tips and producing a quality demo reel, to working with an agent and the differences between union and non-union work. I have the 3rd edition of this book.
If you've ever read anything about working professionally in voice-over, then a lot of the information in Clark's book will be a review for you, but that isn't a bad thing - fundamentals are always important to remember, no matter the topic, and the way Clark breaks down each aspect of the field makes it simple to learn even if you're just skimming through instead of properly reading it.
Where this book truly shines is that it is so comprehensive as to include a variety of detailed copy samples for the reader to practice the concepts and tips Clark discusses. In books like this, I always prefer to be given that sort of content; I'm not much of a creative type, in that I would never know what to come up with if I had to write my own copy to practice with. It would just feel silly and disingenuous, which pretty much defeats the purpose. Besides, I'm not a professional in the field - Clark has already made a successful career in voice-over, so I can trust that copy samples she provides are representative of what one would see when booking a job.
In addition to sample copy and her own professional insights, Clark includes interviews with other successful voice-over professionals, such as Hal Riney (member of the Advertising Hall of Fame) and Ned Lott (who has worked on features for Disney, Pixar, Universal, etc.). Even if you don't recognize the names of people interviewed for this book, their project lists speak for themselves; these are people whose opinions and advice you can trust, because they have been there and done that, and in most cases, they are successfully continuing to do that (I say most, because not everyone is continuing to work...RIP Hal Riney).
Technological advances are quickly making it more convenient for industry outsiders to actively pursue voice-over and become reasonably successful, provided you are willing to put in the work to properly market yourself and you are savvy enough to navigate the field - Clark's book gives you all the tools you need to do so; she explains how new technology is changing some aspects of the field, and how one can adapt to these changes. She also discusses marketing strategy and the union more in-depth than many other sources I've looked over.
There is honestly not much more to be said, apart from the bottom line that There's Money Where Your Mouth Is is a truly comprehensive and invaluable resource for beginners to the field of voice-over work, and I highly recommend it. In fact, if voice-over classes or workshops assign reference material, I think this would (or should) probably be at the top of the required/recommended reading lists.