Written at a high level by mathemeticians. Articles contain links to reviews in MathSciNet, which--in some cases--may be more useful than the articles themselves.
Wikipedia can be a useful starting place for topics in math. The quality varies, so consult references to validate. References may also lead to additional valuable information.
The Princeton Companion to Mathematics
by
Timothy Gowers (Editor)
ISBN: 9780691118802
Publication Date: 2008-09-28
The Princeton Companion to Mathematics surveys the most active and exciting branches of pure mathematics. Accessible in style, this is an indispensable resource for undergraduate and graduate students in mathematics.
CRC Concise Encyclopedia of Mathematics
by
Eric W. Weisstein
ISBN: 0849396409
Publication Date: 1998-11-01
The CRC Concise Encyclopedia of Mathematics is a compendium of mathematical definitions, formulas, figures, tabulations, and references. (NOTE: In print in the science library reference section).
The Princeton Companion to Applied Mathematics
by
Nicholas J. Higham (Editor)
ISBN: 9780691150390
Publication Date: 2015-09-15
Presents the major ideas and branches of applied mathematics in a clear and accessible way.
Encyclopedia of Applied and Computational Mathematics
by
Bjorn Engquist (Editor)
ISBN: 3540705309
Publication Date: 2015-11-23
EACM is a comprehensive reference work covering the vast field of applied and computational mathematics.