The RFC are the authoritative documents describing how things work. Everything else is just a summary or distillation.
I prefer to start with the summaries that other experts write, since many RFCs amend or supersede other RFCs, or some field experience may clarify ambiguous parts of an RFC. However, in the end, the RFC provides the final say on how things work.
Even if you are not an expert, the introductory parts of the RFC are usually accessible, even to the networking novice, explaining the problems leading to the RFC and rationale for the design. The details are mostly of interest to the implementer and diagnostician.