The New Solar System
Summarizes what we've learned from interplanetary explorations in the last 25 years. My primary reference for The Nine Planets.
Encyclopedia of the Solar System
A more scholarly introduction the planetary science for those who want to dig a little deeper.
The Compact NASA Atlas of the Solar System
This 'road map' of the solar system contains lots of maps and data as well as photos.
Universe through Binoculars
A personal tour of the universe using nothing more than a pair of binoculars.
RL = 2.456*R*(p'/p)^(1/3)
where p' is the density of the planet, p is the density of the moon, and R is the radius of the planet. (here's the math)