Network Security with OpenSSL: Viega, Messier, Chandra
Posted by Mark on December 14th, 2005
This is not only a great guide to OpenSSL, but also a great guide (even intro) to PKI and cryptography in general. While it only scratches the surface (it is no replacement for Applied Cryptography), it does so in a way that help you understand the applicability and real world use of the abstract concepts than make up this field.
That aside, it is a pretty complete guide for anyone wishing to use OpenSSL for any one of the several hundred security and cryptography functions it performs.