Encyclopaedia of Mathematics
Ref. QA5 .M3713 1988
Articles written by experts about almost every topic in mathematics across nine volumes, with further citations.
CRC Concise Encyclopedia of Mathematics
Ref. QA5 .W45 2003
Contains short articles with references about most topics in mathematics. A great starting place for background research and finding citations.
Encyclopedia of Applied and Computational Mathematics
Springer ebook containing definitions, explanations, and citations related to most topics in applied math.
Sometimes browsing the shelves is helpful. Some relevant call number ranges are:
QA 1+ General Math
QA 8.9+ Mathematical Logic
QA 21+ Math History
QA 75.5+ Computer science
QA 150+ Algebra
QA 276+ Mathematical Statistics
QA 297+ Numerical Analysis
QA 299.6+ Analysis
QA 440+ Geometry