The OSSTMM isn't the easiest or most fun document to read but it's full of advanced security information that's practical and relevant. It's also the best-known operational security manual on the planet with about half a million downloads each month for one particular reason: those who figure it out have a distinct security advantage, as its instructions are about a decade ahead of the current buzz in the security industry.
The goal of the OSSTMM is to put forth a standard for internet security testing. It is intended to form a complete baseline for testing that, when followed, ensures a thorough and comprehensive penetration test has been undertaken. This should enable a client to be convinced of the level of technical assessment independent of other organization concerns, such as the corporate profile of the penetration testing provider.