Here are a few suggestions. A Handbook of Nature by Anna Comstock, Fun with Nature and More Fun with Nature by Laura Evert, Nature's Art Box by Laura Martin, Pocketful of Pinecones by Karen Andreola, Red Leaf, Yellow Leaf by Lois Ehlert, Take a Tree Walk by Jane Kirkland, Keeping a Nature Journal by Clare Walker and One Small Square (series) by Donald Silver. Here are a few websites as well. Nature Journals, Acorn Naturalist, Backyard Nature, Plant Identification Label and eNature. There are also several nature encyclopedias available.
We really enjoy nature studies here. I hope you can fine something that will be of help.
Have fun!
Cheers!:)