This is a nice (and one of the few) printed resources available on the subject of OpenSSL. It’s a bit dated with a publishing date from 2009 but the general concepts are still the same. You’d certainly do well to read up on the latest versions of the toolkit since there have been a number of security remediations since this book was put out.
I won’t belabor the internals since you can read the table of contents online. I found it to be a good resource for such things as the OpenSSL command line interface, PKI info, and public key algorithms. Again, the latter is dated since it covers topics like Diffie-Hellman which is a weak cipher that most companies are dropping support for in their web tier’s cipher suite.
While you can find the latest info online, I sometimes like to have a physical reference that I can carry around with me. I guess I’m a little old school in this regard but as much as I love my tablet sometimes nothing beats having a real book in your hands.