This is not an example of great mathematical writing, but definitely something to know and probably even more useful: Paul Halmos' brilliant essay "How to write mathematics", which you'll find for example at http://retro.seals.ch/digbib/view?rid=ensmat-001:1970:16::59&id=browse&id2=browse2http://i11www.iti.kit.edu/~awolff/lehre/scientific_writing/h-hwm-70.pdf.
