Elements of Basic Category Theory, by AlfioMartini. Maybe this should be CategoryPaper (there doesn't appear to be a separate print edition; nor does this work have an ISBN that I can find) but at 160 pages I consider this one a book. Plus, it is written more for pedagogical reasons rather than as a research paper in its own right.
Available from CiteSeer at http://citeseer.ist.psu.edu/310819.html
See also CategoryTheory