It's a bit late to add this, but I realize that I wasn't entirely clear above. When I say "results in maths (that) require an entire book length treatment" I don't mean a proof that is inherently book length. I mean, a proof that justifies (one or more) book length "downstream" treatments that explain, clarify, expand, etc., the base proof.
Going back to Gödel's Incompleteness Theorem(s) for example... yes, the original proof is not that long, but for whatever reason, people have felt compelled to write entire books on the topic, after Gödel, to say more about the proofs. That kind of thing is more what I was thinking of, FWIW.
Going back to Gödel's Incompleteness Theorem(s) for example... yes, the original proof is not that long, but for whatever reason, people have felt compelled to write entire books on the topic, after Gödel, to say more about the proofs. That kind of thing is more what I was thinking of, FWIW.