Ecosystems, Conservation, Ocean Studies, GIS, Hydrology - you can download free digital textbooks on all this and more from the Earth Science section of National Academies Press. Price listed is for delivery of paperback copies.
An animated, video, html, lesson about plate tectonics that includes Wegner's hypothesis, the Theory, the driving forces, the three plate boundaries, convection curreents, hot spots, and plate tectonic reconstruction.
Hosts more than 3,100 maps, 1,400 reports,and 600 Geographic Information System (GIS) datasets representing more than 150 years of history. Digital copies (in pdf format) are available for free download.