Here’s a great link for summer reading.  The AMS has free online books, and at this link you can find books on the history of math (just scroll down to “Free Online Books”)

 

(http://www.ams.org/samplings/math-history/math-history)

Included in this list are Mathematics into the Twenty-First Century and A Century of Mathematics in America (3 volume set).  There are also free online resources, such as columns written on mathematical history.