Mechanics of Materials by F.P. Beer, E.R. Russell Johnston Jr. and J.T. DeWolf

Mechanics of Materials is the book I used for my undergraduate course in strength of materials and it helped me understand the basics, and hence my selection. 

The outline is as follows:

Ch. 1: Introduction -  concepts of stress

Ch. 2: Stress and strain - axial loading

Ch. 3: Torsion

Ch. 4: Pure bending

Ch. 5: Analysis and design of beams for bending

Ch. 6: Shearing stresses in beams and thin-walled members

Ch. 7: Transformations of stress and strain

Ch. 8: Principal stresses under a given loading

Ch. 9: Deflection of beams

Ch. 10: Columns

Ch. 11: Energy methods 

This book does not relate directly to the material covered in ES 240, but is very useful in gaining basic understanding of and some physical intuition for various simple loads. Therefore, the book does not complement the course, but rather provides a fundament one can go back to for clarification on basics if in doubt. Also, many of the cases treated in the book are simplifications of the cases we deal with in ES 240, and I think it is useful to go back to the simple versions every now and then and draw connections between the general case and the simplification.

The feature of the book that I particularly like is that it is very graphic. This might sound trivial, but one picture speaks a thousand words and a good 3D drawing really helps me  visualize the physical situation. More advanced textbooks, such as Theory of Elasticity by Timoshenko and Goodier very often neglect the importance of graphics in learning (which is understandable, but nevertheless clearer drawings would sometimes make the readings a lot easier).

Pawel