More on the parent website:

This book is one of four free books available from learningGEOSCIENCE. The site started out independently hoping to replace textbooks with online learning modules. Then in 2004, it entered into an agreement with the European Association of Geoscientists and Engineers (EAGE) to become their online learning portal. The site, although confusing to navigate, is packed with nearly 100 flash learning modules and a handful of longer (8-10 hour) courses.

Unfortunately, to use the vast majority of these resources (except for the four free books linked here), you must be a member of the EAGE. One upside is that they appear willing to work with individuals and institutions to make this material affordable, so it’s possible that a university could get a bulk license for a particular class for a per-student cost far below that of standard textbooks. A professor with access to the full site would have a fantastic tool, far better than any one textbook. Another upside is that EAGE offers a number of free student memberships every year to qualifying students. For those students who can’t get in for free, the yearly 25 euro membership fee is a far better deal than a $120 textbook.