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.

Indispensable guide to writing security applications using the OSF DCE framework. Well laid out with plenty of sample code and concise explanations. This book was my constant companion for about two years, and never let me down.