Fundamentals of Piano Practice: Great book about practicing more efficiently and using techniques that noticeably improve your playing abilities by research Scientist/amateur pianist Chuan C. Chang.
Download the book free:
Buy a hardcopy version:
Songsterr: Basic level of Songsterr is free. Thousands of songs in all genres written in guitar tablature. Songsterr also has a player function so you can hear how the notation should sound.