The Foundation Book is the newest one. I bought it accidentally when I started my lessons and I was very glad my teacher said it was the wrong one. The approach totally turned me off, which is just an individual thing and not meant to put the book down. It may very well work for others. The book my teacher wanted me to buy was the Technique book, I believe the oldest one of the series, although it wasn’t the oldest edition. It has a totally different approach from the Foundation book. However, it, too, is a little annoying as Shearer harps on with a lot of “thou shalts”. But used with a teacher the music works in a very elementary and progressive manner. I noticed my teacher modified some of the “thou shalts” so if using the book on your own use common sense to dilute some of the black and white advice. When I got past the first half of the book my teacher started giving me music from the RCM Preparatory book as well. The Shearer book is a little unexciting, but a good resource.
The first book is how to play, mostly working on right hand development. The second book gets into music theory. It looks very wordy. I have it but so far my teacher has not started using it. Maybe because I told him I don’t like the book. It is printed very poorly and my old eyes have trouble with the unclear print. I have a piano book for adult learners that explains elementary theory much more clearly so maybe that is why so many are centred around piano. It’s easier to visualize. I didn’t think the first Technique book would be good until I spent a year with the thing and discovered what it could do for me. The same might or might not be true for the second book. It seems to be a good resource but I wouldn’t want it to be my only one.