Crypto Drop Box.
http://www.und.nodak.edu/org/crypto/crypto/.
Contains figures from Hill's patent application,
pictures of a German Enigma machine, and other material. Maintained by the
American Cryptogram Association.
Sarah Flannery with David Flannery,
In Code: A Mathematical Journey.
Workman Publishing, 2001.
A very engaging and readable account by Sarah
Flannery, whose high school project on cryptography (done when she
was 16) won the 1999 European Union Young Scientists contest.
Part memoir, part puzzle book, and part mathematical exploration, and
all very entertaining.
Martin Gardner,
Codes, Ciphers and Secret Writing.
Dover Publishing, 1984.
Explains in simple terms how to encode and
decode messages in various ciphers, describes coding machines, and
gives formulas for invisible ink. Appropriate for middle school
students.
David Kahn,
The Codebreakers; The Comprehensive History of Secret Communication
from Ancient Times to the Internet.
Charles Scribner's Sons, 1996.
A massive (nearly 1200 pages!), but very
readable historical account of cryptography. A classic.
Robert E. Lewand,
Cryptological Mathematics.
Mathematical Association of America, 2000.
Covers the essential number theory required to
understand various encryption schemes. Requires mathematics at
about the level of a good high-school student.
Scott Sutherland,
fsqFsHn sGGousG (Secret Messages).
http://www.math.sunysb.edu/scott/Book331/crypto.html.
Part of my own text aimed at doing mathematics
with computers, for math majors in their junior or sophomore year of
college. Uses the Maple programming language.
Encryption, article in Wikipedia.
http://en.wikipedia.org/wiki/Encryption.
A good online article with more details about most of
the material here, and links to much more.
9 Adieu
V'ir Tbg n Frperg 
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