Search for books on voice over and you'll find plenty to choose from. Quality and relevance vary from book to book, but one of the best ones I have discovered over the last couple of years is Joan Baker's Secrets Of Voice Over Success.
Rather than a technical manual or a step by step how to do book, Joan approaches the subject in a simple but effective way. She collars America's top voice artists and lets them speak for themselves. The result is an interesting formula with the likes of Don La Fontaine, Joe Cipriano, Dave Fennoy and many others revealing how they got their start, their methods and tips for people new to the industry.
Don La Fontaine and Jo Cipriano appeared on the Paul Harris show to publicise the book, the proceeds of which go to an Alzheimer's charity, giving a lively, interesting and amusing insight into their work. Here the interview here.