"Going Around the Sun: Some Planetary Fun" by Marianne Berkes.
It's a gorgeous picture book, in verse, with little factoids about the planets on each page, plus a more detailed section at the end. It's even up to date on Pluto's status. Be sure to get the paperback version. It has some extra tips from the author and illustrator (websites, books, activities, etc.).
"Planets - A Solar System Stickerbook" by Ellen Hasbrouck
It's also in verse, has good colorful pages and is also up to date on Pluto (something my son is very particular about!). The stickers don't last long, and the information about Jupiter, Saturn and Uranus's moons isn't quite current, but it's still a good read for younger kids.
Let me know if you're interested in space songs too. I've been compiling an iTunes playlist for my son. You wouldn't believe how many solar system songs are out there (especially "ode to Pluto" ballads).
