The Piano Handbook by Carl Humphries
It's a very good, indepth learn-at-home piano course that's fairly simple to follow. I'm currently studying it now.
It's much easier to follow & you won't get lost in it like you will w/the Dummies series or Idiot's guides.
It has to be the absolute best I have found so far & I've went through several at-home study book courses already.
Good luck!