Online virtual textbooks and encyclopedias

This Dynamic Earth: The Story of Plate Tectonics, a U.S. Geological Survey online publication.

The Introductory Geoscience Virtual Textbook, This resource represents an online reader and reference specifically designed to accompany an introductory geoscience course at the undergraduate level. It was compiled as a subset from the vast resources available online through DLESE.

The Physical Environment: An Introduction to Physical Geography, online physical geography textbook by Michael E. Ritter.

Fundamentals of Physical Geography, an online physical geography textbook.

The Encyclopedia of Earth, an electronic reference about the Earth, its natural environments, and their interaction with society. The EoE is a free, expert-reviewed collection of content contributed  by scholars, professionals, educators, practitioners and other experts who collaborate and review each other's work. The content is presented in a style intended to be useful to students, educators, scholars, professionals, as well as to the general public.

The Encyclopedia of Life, a free, online collaborative encyclopedia intended to document all of the 1.9 million living species known to science. It is compiled from existing databases and from contributions by experts and non-experts throughout the world.