I can't say that I am aware of any one book that goes into great depth on the engine. If you buy several books, you will stand a better chance of learning about the car. If you don't have the Bentley Manual, it is a good place to start.
Try searching the web for engine information. It doesn't necessarily have to be for TR6, any straight four, six or eight articles will give you the basics on how an engine works.
If you don't belong to a local Triumph Club, join one. Members are generally more than happy to help out people with an interest in learning. Clubs will offer you more opportunity to hands on experience, which is better than any book.
And if you regularly read this forum and post questions along the way, you should feel pretty comfortable with knowledge gained in time.