Driving Awareness, Adoptions, and Affordability
I've written a free / open source book for an "introduction to proofs" course called "A Gentle Introduction to the Art of Mathematics" (GIAM for short). GIAM is licensed under the GNU Free Documentation License and is freely available as a PDF at www.southernct.edu/~fields/GIAM/. Also at that site are a multitude of solutions and hints for exercises and the LaTeX source code for the book. GIAM has chapters on logic, set theory, relations and the cardinality of sets interspersed with chapters on various proof techniques.