The book "An introduction to group theory"
by Anthony Gaglione (Prof of Math at the US Naval Academy)
is public domain. It was written as part of an summer job 
working for NRL. (Supported by the NRL. 
Author’s address: 
Mathematics, Department, 
U.S. Naval Academy, Annapolis, 
MD 21402.
amg@usna.edu)

The book 
Elements of the Differential and Integral Calculus, 
by William Anthony Granville and Percey F. Smith. 
Boston: Ginn and Company, revised ed., 1911. 
is public domain. (The copyright was not renewed and so 
copyright protection expired.) It is also available on the wiki:
http://en.wikisource.org/wiki/Elements_of_the_Differential_and_Integral_Calculus

The book "Introduction to Methods of Applied Mathematics" 
by Sean Mauch (notes from a cource at Cal Tech) has been placed in the
public domain by the author. It is also available at
http://www.its.caltech.edu/~sean/book/unabridged.html


Project Gutenberg's The Foundations of Geometry                       
 by David Hilbert                                                      
This eBook is for the use of anyone anywhere at no cost and with      
 almost no restrictions whatsoever.  You may copy it, give it away or  
 re-use it under the terms of the Project Gutenberg License included   
 with this eBook or online at www.gutenberg.net     

  
The book Vector Calculus, by Michael Corral (Schoolcraft College),  
is distributed under the terms of the GNU Free Documentation License, 
Version 1.2. http://www.gnu.org/licenses/fdl.html

Coding Theory and Cryptography: From Enigma and Geheimschreiber to 
Quantum Theory, which I edited, is published by Springer-Verlag. 
This is now out of print and will not be reprinted (as far as I know). 
It is not open source, but the publisher has allowed me to
distribute all articles via the www (see the file
cryptoday_permission.txt). Their are in the cryptoday directory.

 
The books by David Santos (http://faculty.ccp.edu/faculty/dsantos/)
in the "santos" subdirectory are distributed under the terms of the 
GNU Free Documentation License, Version 1.2. 
http://www.gnu.org/licenses/fdl.html