You may like N. Y. Vilenkin's Stories About Sets, although it's less formal than you could expect:http://www.amazon.com/exec/obidos/tg/detail/-/012721951X/qid=1033569397/sr=1-5/ref=sr_1_5/104-8793231-9790359?v=glance
http://www.amazon.com/exec/obidos/tg/detail/-/012721951X/qid=1033569397/sr=1-5/ref=sr_1_5/104-8793231-9790359?v=glance