Eric Horst is well-known as a climbing coach and author, and his latest book Learning to Climb Indoors is his latest and greatest. We still have a copy of his 1997 How to Climb 5.12 on our bookshelf, and it still holds up well.
LTCI is really a fantastic one-volume reference for beginning and intermediate climbers, and even advanced climbers will get a lot out of it because it does a great job of systematizing a complex sport. Chapters two and three cover gear and safety and chapter four sets the stage by talking about how to learn in general terms, e.g. concepts and tactics to keep in mind as you strive to master the skills.
Then it's off to the races with five solid chapters on basic and advanced techniques, lead climbing, the mental game and conditioning. All are very well thought out and presented, for example the basic techniques chapter is built around five clear fundamentals and the advanced chapter around six.
The conditioning chapter is very climbing specific with instructions for building finger and tendon strength and endurance. Another nice feature of this book is that while it is targeted at adults it doesn't ignore kids, for example pointing out that kids are better served in full body harnesses, and there's even a full chapter on youth climbing. At the end there's a great summary chapter on building a home wall and on transitioning to outdoor climbing. If you own only one book on training to climb indoors, this should be it.