“#X.509 Style Guide” by Peter Gutmann
▻http://www.cs.auckland.ac.nz/~pgut001/pubs/x509guide.txt
Do not be misleaded by the title: it is not a style guide but an implementation guide for the poor programmer forced to implement the cyptographic certificates X.509 standard. X.509 is one of the masterpieces of #ITU: bloated, badly written, self-contradictory and unusable as-is (you have to define profiles, which are restrictions of the standard, to do anything useful; for instance, the Internet relies on the #PKIX profile).
A great reading for the persons who write standards, and for the programmers. If you want to have an idea of how hell looks like, read this text and you will be glad to have a dull and stupid job, rather than being asked by your manager to process X.509 certificates.