Authors who have written many wonderful nature-themed books for younger readers, along with a sample title:

Caroline Arnold:
Birds: Nature's Magnificent Flying Machines

Sneed Collard III:
The Forest in the Clouds

Holling Clancy Holling: Pagoo

Wendy Pfeffer:
A Log's Life

Laurence Pringle:
An Extraordinary Life: The Story of a Monarch Butterfly

Donald Silver:
One Small Square: The Night Sky

Doug Wechsler:
Bizarre Bugs

And a sprinkling of authors/titles for older readers:

Henry Beston:
The Outermost House: A Year of Life on the Great Beach of Cape Cod

Christopher Cokinos:
Hope is the Thing with Feathers

Sy Montgomery:
The Wild out Your Window

Scott Weidensaul:
Mountains of the Heart: A Natural History of the Appalachians

Plus, some good web sites:

National Wildlife Federation
http://nwf.org

National Audubon Society
http://www.audubon.org

The Sierra Club
http://sierraclub.org

Bill Nye, "the Science Guy"
http://billnye.com/flash.html

Science News for Kids, of Science News magazine
http://www.sciencenewsforkids.org