There are bigger triathlon books, and there are books with more detailed training schedules. There are books to hone your swimming, and books to make you run faster, but there is probably no better book for starting the sport of triathlon than Eric Harr's
For starters, Harr does a decent job as a motivational writer. This is no small feat for an accomplished athlete writing to novices. Harr manages to inspire without condescending. He gets you excited about the sport.
Next, he sets the stage. He goes into the basic equipment required to train for and complete the race. He gives estimates regarding costs and recommendations as to specific brands he has used (this was probably the only part of the book that felt dated, given the 2003 publication). He also asks you to give realistic assessments of your fitness and your race goals. These include simply completing the race, trying to complete a longer distance than previously, or even competing to win (more about that later). He helps a reader plan a training schedule that matches their fitness level with their goal while allowing time to have a life outside of the sport.
This is followed by more detailed discussion of the various stages, with chapters on swimming technique and drills, biking (conditioning, equipment, and safety) and of course running. He discusses the importance of heart rate monitors and interval training, and he covers when to push your training, when to back. He doesn't particularly go into the science behind the training, but his recommendations are similar to other writers in the field. Above all, he strives to keep the training schedule compatible with achieving athletic success within the time constraints of the modern recreational athlete.
He puts it all together in sections that describe workout plans. These are tailored to fitness level and personal goal. They include recommendations as to sport, duration, and intensity (divided into zones I to III in increasing order of difficulty). These plans are outlines, leaving some details to the athlete.
This is the recommendation for the 3rd week of a 6-week plan for a "Fitness Enthusiast", the 3rd highest in his 4-level hierarchy of fitness:
Week 3: Moderate Intensity
Monday: rest
Tuesday: swim for 30 minutes in zone II
Wednesday: run for 30 minutes in zone III; strength train for 40 minutes
Thursday: swim for 30 minutes in zone I; bike for 60 minutes in zone III
Friday: rest
Saturday: Brick workout - bike for 50 minutes in zone I, then run for 20 minutes in zone II
Sunday: strength train for 20 minutes
Other miscellaneous chapters include discussions of injury prevention and treatment, nutrition, and motivation. An often overlooked aspect of endurance training is strength training, but I found that chapter to be very up to date. The exercises incorporate aspects of modern strength training, such as core training, and he discusses home as well as gym workouts.
Now, is this the only book a triathlete would ever need? That depends. If your goal is to run the occasional race, even to improve on a previous time, then this will probably be enough. Experienced triathletes will probably find it aimed too far below them. The Triathlete's Training Bible by Joe Friel would probably be a better bet for an athlete trying to move from the middle of the pack to the awards table. That's not a knock on this book; it's just pitched at a different audience.
In summary, anyone interested in taking up the sport of triathlon would have a hard time doing better. Start here, end at the finish line.