I have used this as a core part of train the trainer programmes for ten years or more. It is a practical, easy to read guide to the realm of adult learning and adult teaching - like it says on the tin. Too often academic books focus on theory and forget that teachers have to be learners first: understanding the way we learn ourselves is key, in my view, to being able to encourage others to learn.
So if you want to train (teach if you must) then learn. And if you run train the trainer courses, use it as the number one handbook.