You can also find books on math history by doing keyword searches in DELCAT, the University of Delaware Libraries' catalog. The Library of Congress Subject Headings, used in DELCAT, has several terms that enable you to identify books on the history of mathematics. Video tutorial to using DELCAT.
Sample subject searches in DELCAT:
1) Mathematics History (UD Library holdings only)
Search syntax => Subject contains exact phrase Mathematics History
2) Mathematics History (UD and libraries worldwide)
Search syntax => Subject contains exact phrase Mathematics History
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