This is a selected list. If you don't see the title you need, check MNCAT.

Encyclopedia of Mathematical Physics
published by Elsevier, 2006
Encyclopaedia of Mathematics
published by Springer, 2001
Encyclopedia of Statistical Sciences
published by Wiley, 2005
Handbook of Mathematics
published by Springer, 2004
Concise Oxford Dictionary of Mathematics
published by Oxford, 2002
Eric Weisstein's World of Mathematics
an open-access, user-contributed encyclopedia
Lecture Notes in Mathematics
vol. 1649, 1997- present
electronic books published by CRC Press
Distributed Digital Library of Mathematical Monographs Collection
open-access, searchable full-text historical math books from Cornell, U. Michigan, U. Goettingen
Online Books: Mathematics
links to open-access e-books, in subject/call number order