Ball, W. W. Rouse,
Mathematical Recreations
and Problems
of
Past and
Present Times
, London: Macmillan, 1892. (Very popular, contains considerable history.)
Beckmann, Petr, A History of ¹(Pi), New York: The Golem Press, 1971. (The mathematical level of this book is flexible and there is plenty for readers of all ages and interests.)
Bell, E. T.,
Men of Mathematics,
New York: Simon and Schuster, 1937. (Very readable biographical accounts assume relatively little mathematical background.)
James, Glenn, and R. C. James,
Mathematics Dictionary,
3rd ea., New York: Litton Educational Publishing, Inc., 1968. (Useful.)