We have a couple of those books... we introduced them to the kids probably almost a year ago.Savannah really liked the books and I think the theory works really well- I love how it's spelled out to the kids in such a visual way. Sawyer doesn't quite grasp the idea as well as I'd like yet,but then again we haven't talked about it in awhile. It might be time to look through those again with him.I think it's definitely one of those things that at this age depends on the kid. Sawyer- meh, not so much... but I told my friend about them and she borrowed them and her three year old really took to the idea (he's younger than Sawyer). I could see P really taking to it.
My kiddos love it and it works well for us! Ayden learned it at school but brought it home and we started using it at home. We had to make it visual for maddie...using marbles and a jar...hope it works for you!