In response to an inquiry from luag about recommendations for books [I] have read or found helpful about the process of learning to play, my current preferred teaching curriculum:

While some theory helps, the process of learning to play an instrument is a lot like sports: you just have to practice.

I like teaching out of a combination of:

1) a lesson book with progressive songs (gradually increasing musical vocabulary and difficulty)
2) a theory book (understanding why songs tend to be structured how they are, among other things)
3) a technique book (strengthening weaker fingers and learning posturing that is conducive to various types of movements)
4) a classical or other fun book (songs written by masters rather than someone just trying to demonstrate a principle: they tend to sound better and be more fun to play)

As far as which books, I learned on the Thompson series myself, but prefer the Faber lesson book series, which is what I teach out of. I'm currently experimenting with different theory and technique books, and have not yet settled on a favorite. Which supplementary book to get depends a lot on what the student is most interested in.

I recommend that adults use lesson books designed for adults (also called accelerated in some series). They tend to move at a much faster pace whereas the child books tend to have a fair bit of practice and repetition of concepts that many kids need in order to fully grasp the concepts but that most adults will find mind-numbing ;) (older or exceptionally smart and/or diligent kids can also use accelerated books of course)