Years ago I worked for some architects who showed me how to manage an ISBN system - organizing books by subject matter, like they do in libraries, with very detailed sub-divisions. With a small amount of poetic licence, that's how I have tried to organize my own books, with notable exceptions.
In my bedroom I have two whole shelves containing books I have the firm intention of reading quite soon - very mixed subject matter! Then a few shelves of reference books, as I work on the computer in that room, and some non-fiction like psychology, sociology and philosophy. And a whole bookcase on true crime and forensic science.
In my hall I have a book case of poetry and drama, language learning (I've had a dab at the languages of almost every country I have visited), and my overflow of fiction paperbacks.
In my lounge I have hobbies - lots of books (many of these are oversized) on gardening, arts and crafts , cookery and lots more novels. I have a top shelf for leatherbound decorative antiquarian books, which I love to see, but don't actually read these days.
In my kitchen - just a few of the recipe books I am most likely to use.
Having run out of space, I then have numerous boxes of books, all containing just one subject type, such as politics, military and spies, science fiction, technical, business, and so forth, all duly labelled so that I can put my hands on them easily. The cardboard boxes are covered with a nice throw, so that they look less hideous, or piled up in my cupboard under the stairs.
I'm a real magpie when it comes to books. I don't have anything especially "foundational" as you put it, but maybe reference books on computer or language subjects and a few classics and prize-winning novels,