Pentesting isn't about knowing RFCs by heart. If there is a specific information you need for a specific test, you can always just look it up.
It's much better to have broad fundamental knowledge, and know where to find specifics, rather than to attempt to cram as much specific knowledge into your head as you can.
The question "Which books should I read when starting Pentesting?" is very opinion-based, and will not be answered, as those answers would be outdated very quickly. One resource that comes up often is the OWASP Top 10, as those are vulnerabilities that come up very often, and understanding those will help you quite a bit.
Don't be afraid of asking well-formulated questions regarding your understanding of specific topic, even if they are not attached to a real-world problem.