Dave Witte Morris wrote:This free undergraduate textbook provides an introduction to proofs, logic, sets, functions, and other fundamental topics of abstract mathematics.  It is designed to be the textbook for a bridge course that introduces undergraduates to abstract mathematics, but it is also suitable for independent study by undergraduates (or mathematically mature high-school students), or for use as a very inexpensive supplement to undergraduate courses in any field of abstract mathematics.

The book is being released online with a Creative Commons license (Attribution-NonCommercial-ShareAlike 2.0).  Although not in final form, it has already been used as a textbook for several semesters at the University of Lethbridge.  The preliminary version of November 2013 is approximately 200 pages.