If you would have the power to change any Emacs-default setting, what would that be and why?
What kind of setting do you think would be useful for a lot (or a majority) of Emacs-users?
One of my examples:
(fido-vertical-mode t)
Because seeing a list of candidates as you type and being able to choose from it with meaningful key-bindings is way more convenient and efficient than the tab-completion-system (I think this claim is backed by the way all those external packages work; helm, ivy, vertico). This is especially suited for a default because you can still use the tab-completion-system with it. So nobody needs to change habits.
I don't about it's page count, but the goto-place for your need seems to be the "Introdution to Programming in Emascs Lisp" which is reachable from emacs with
C-h i m Emacs Lisp IntroIt is structured nicely, so you should be able to find the relevant parts easily from the table of contents, you don't have to read it all. None the less I would recommend to read as much as possible, because it's very informative and (in my experience) well worth it.