"Paul Halmos's writing affected the professional lives of nearly every mathematician in the latter half of the twentieth century, and it will continue to influence the profession for years to come." -- J. Ewing, Notices of teh Amer. Math. Spc. "An exposition which is always fresh, proofs which are sophisticated, and a choice of subject matter which is certainly timely." - Bulletin of the American Mathematical Society. "In one hundred and nine well-packed pages one finds an exposition which is always fresh, proofs which are sophisticated, and a choice of subject matter which is certainly timely. ... Most pages exhibit a zest for play as well as work which is refreshing." - E. R. Lorch (Bull. Amer. Math. Soc.)