''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

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See also CategoryTheory
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CategoryOnlineBook