There is a rich literature associated with astronomy and planetary science. The trouble is that, the longer or more specialized the book,the faster it goes out of date. On the other hand, some (not all!)websites are frequently updated. To help you discover more about planets, I suggest a few of the best books and several appropriate entry points to the internet.
General
J. K. Beatty, C. C. Peterson, and A. Chaikin (eds.), The New Solar System , 4th edn. (Sky Publishing Corporation and Cambridge University Press, 1999). This covers the lot. Each chapter is written by a specialist author. Badly dated in parts, but it remains a highly accessible classic.
I. Gilmour and M. A. Sephton (eds.), An Introduction to Astrobiology(Cambridge University Press, 2003). Updated in 2007, this is the second of two volumes based around an Open University course on planetary science, written at early undergraduate level. This one covers life, Mars, Europa, and Titan as potential habitats, and exoplanets. New edition expected 2011 .
N. McBride and I. Gilmour (eds.), An Introduction to the Solar System (Cambridge University Press, 2003). Updated in 2007,this is the fi rst of two volumes based around an Open University course on planetary science, written at early undergraduate level.It covers all the major components of the Solar System, except the Sun. New edition expected 2011.
S. A. Stern (ed.), Our Worlds: The Magnetism and Thrill of Planetary Exploration (Cambridge University Press, 1999). Easy but informative reading. Each chapter is a personal account by one of the leading practitioners.
D. A. Weintraub, Is Pluto a Planet? (Princeton University Press,2007). If you’ve read this far, then you already know the answer to the question posed by this book’s title. However, it covers much more than that, being an historical account of human perception of planets from ancient times right up to the recent squabbles over the classifi cation of TNOs.