Strength of Materials
This is a free, online wikibook, so it is continually being updated and refined. According to the authors, "This book is a first course in the analysis of structures. Although most of the material should be accessible to all students who have had a mechanics course, a previous exposure to Engineering Mechanics would be useful. There are no mathematical prerequisites, though some elementary calculus would be useful in certain sections which can be skipped without affecting the flow of the book."