(Full disclosure: I am the author of the book Cryptography for Visual Basic.)

Currently there are two books available for Visual Basic programmers who want to use cryptography. Both use the Microsoft CryptoAPi as a source of cryptographic algorithms. In my opinion you should acquire both of these books. For more information, see the links at left.

A third book that virtually all programmers use is Bruce Schneierís Applied Cryptography, which was the first comprehensive text on cryptography for programmers. It is getting a little old, but is still extremely useful.