Real World OCaml is peppered with interesting tidbits about the implementations of OCaml that help with understanding the way the language is constructed.
Here’s a quote about the limitations of pattern matching, which helped my understand the deliberation behind its constraints.
You can think of patterns as a specialized sublanguage that can express a limited (though still quite rich) set of conditions. The fact that the pattern language is limited turns out to be a good thing, making it possible to build better support for patterns in the compiler. In particular, both the efficiency of match statements and the ability of the compiler to detect errors in matches depend on the constrained nature of patterns.