I went crazy and splurged on piano books so I can self-teach/practice without a teacher during the summer to get ready for my college piano course next semester. I wouldn't mind taking it multiple times.
Some of the things I've bought were:
A Dozen a Day Preparatory
A Dozen a Day Play With Ease in Many Keys
Rocket Piano Physical Copy Upgrade (I don't usually go for things like this but I thought it would be worth my time when I bought it)
The Musician's Way, A Guide to Practice, Performance, and Wellness
Hanon's Virtuoso Pianist in 60 Exercises
Jumbo Easy Piano Songbook: 200 Songs for All Occasions
Big Book of Beginner's Piano Classics: 83 Favorite Pieces in Easy Piano Arrangements
Musician's Practice Planner (I don't plan to use this until I actually get to that class)
My First Book of Classical Music: 29 Themes by Beethoven, Mozart, Chopin and Other Great Composers
I already had a few other books under my belt, including a couple of children's music books and Alfred's Self-Teaching Piano Course for Adults, and Synthesia too. For some reason, it's always been that the more money I spent on something the more I would take it seriously. I really wanted to get back into practicing but my theory and tech is extremely... I wouldn't really say off, but it's not where I want it to be. Not only that, but I don't know many songs.
I know Chopin's Prelude Opus 28 No. 4, and an easy short version of Tchaikovsky's Swan Lake. I wanted to be able to have more than that as well.
I want to be one of those people with a real passion for the music they play. I don't want practice to be a chore anymore.
I'm in the process of trying out these different books/supplements, so hopefully we can see which is the best for improvement.