Study Guide for a Beginning Course in Ground-Water Hydrology
According to Textbook Revolution, a student-run site “dedicated to increasing the use of free educational materials by teachers and professors,” "The USGS wrote this study guide for Hydrology in 1990 in an attempt to boost the number of hydrologists. Although it’s intended to be used in conjunction with a textbook, the study guide is so thorough that it could probably be used alone. The book is a PDF scan of the original typewritten pages, which means that you can’t search it, but there is a pretty good table of contents and it’s well-organized. There are plenty of exercises to help students practice. An excellent resource for anyone struggling with a tough subject."