I'm currently working on a children's activity book about the desert for Nomad Press. My research is turning up a lot of great websites, among them Wayne's Word, an online Textbook of Natural History written by Wayne. P. Armstrong. The website is a supplement to Professor Armstrong's general biology and botany courses at Palomar College in California.
Armstrong is retired, but continues to teach two of his courses -- Plants and People (Botany 115) and General Biology (Biology 101)-- online, with no meetings on campus. He writes that both courses are based on the thousands of pages of lecture notes that were laboriously placed on blackboards and whiteboards for more than thirty years, along with more than 2,300 photo images and illustrations.
That's a lot of material, all available online, for free. While I've only skimmed the site and read a few articles (about desert micro-organisms), it seems to be accessible, well organized and nicely illustrated. Check it out!