When searching for online references, be sure to try the encyclopedic database Credo Reference, or Encyclopedia Britannica. Limit your searches by subject, title, classification system, or material type.
Listed below are a few of the reference resources available for viewing. Click on the title link to access the e-book content (when available).
Planet Earth : the complete series
Five Kingdoms : an illustrated guide to the phyla of life on earth by 
Oxford Companion to the Earth by 
The Tree of Life : a phylogenetic classification by 