Hi there. I'm new to the world of homebrewing so apologize if this has been asked or if it's a rather dumb question.

Anyway my question is this: is there a way to make a homebrew based on content shared through my master's books? I'd like to test some changes by myself without bothering the master (and also help him study the homebrew system since he's new as well). I figured I might not be able to, since in the homebrew menus none of the books-related content shows up, but I guess asking could never hurt.

No, you have to own it yourself to use it as a template.

Alrighty, that's what I thought. Thanks for the reply!

Another quick question: if I wanted to make a feature that lets you choose between any spell from a given school, how would I be able to do it?
I got inspired by the cleric's Death Domain, which says this: 

At 1st level, you learn one necromancy cantrip of your choice from any spell list. When you cast a necromancy cantrip that normally targets only one creature, the spell can instead target two creatures within range and within 5 feet of each other.

Which basically gives access to any cantrip of the necromancy school. How could I do something on the same lines? As of now, I've got around the problem by manually adding the said spells in a trait with options, but this means I would have to add options whenever a new spell of that school gets added into the game and/or when I buy another one.

The way you did it is the only way to do it currently as far as I know.