Cloud Boutique - Contains photographs of the different types of clouds.

Weather.Com  - Good resource for lesson plans and information related to weather. 

WW2010 - Clouds and Precipitation - Sophisticated information on clouds and cloud types.

Clouds - Filled with  more good photographs

Cloudy With a Chance of Meatballs by Judi Barrett.
A fun, fanciful story of what would happen if food, rather than water, fell from the sky.

The Cloud Book by Tomie dePaola.
A factual book, with humorous illustrations, regarding the science of clouds.  It also includes some myths and folklore regarding cloud formations.

Clouds by Gail Saunders-Smith.
A nonfiction book of clouds.

Cloudland by John Burningham.
A story of a little boy who falls into the clouds and spends some time with the "cloud children".

Little Cloud by Eric Carle.
The story of a little cloud who drifts away from his friends and changes his form into lots of different shapes.

It Looked Like Spilt Milk by Charles G. Shaw.
This book encourages children to use their imaginations and see  different shapes in cloud formations.  It leads in nicely to an art lesson or to a trip outside to do some cloud gazing.