I've never seen any really good expositions of elementary mathematics (middle school or earlier). A good college-level textbook, written for people with an interest in mathematics, reads like a novel or an impassioned essay. The most famous example I can think off the top of my head of is Spivak's Calculus - the author has a vision for the subject, and wants to communicate their passion to the student.
As an example, the rule for multiplying fractions together takes some quite clever and pretty arguments to prove, but in math class and in classroom textbooks, this is just presented as:
Fraction multiplication rule: Multiply the numerators and the denominators together separately.
Looking at the difference in presentation, you'd almost think elementary arithmetic and geometry aren't part of mathematics. Where are the proofs? Is there even a single book in existence that proves the above fact at all?
Are there any books about elementary math written like actual math books? Note that I am not looking for things like "The Number Devil", which (to be fair I haven't read it) you probably wouldn't mistake for an actual textbook. I'm looking for systematic expositions of basic math, covering (and proving!) all the theorems, rules of computation, and definitions covered in elementary to high school (not necessarily all in one book - that's a lot of material).
I don't necessarily want it to be readable by 8 year olds, that might be asking a lot, but it would be nice to have something you could recommend to a bright 12 year old to give them a more solid foundation and inspire more of an interest in math. Something well written, that wants to make the subject interesting and beautiful, is preferred to a dry going-through-the-motions proof compendium, but of course that's basically like saying "make sure the book is actually good".