Since I had a look at a couple of them, I suppose I might as well provide reviews.
"Introduction to Proofs and the Mathematical Vernacular" -- this is actually pretty good, and I kinda wish I'd had a copy of it at the start of my university mathematical education, if only to teach me what the hell a "Lemma" was, exactly. It's also fun to read.
"21st Century Physics Flexbook" -- very odd. Admittedly intended as a K-12 textbook rather than a university-level textbook, but not much of a way to learn physics even for the kids. Virtually no mathematics whatsoever, let alone calculus. A random grab-bag of topics from "gravitation" (with an attempt to explain relativity which would be more confusing than enlightening) to "nanoscience" and "biomedical imaging". Basic mechanics finally comes in at the end as an afterthought. Not recommended to anyone.
"Introduction to Proofs and the Mathematical Vernacular" -- this is actually pretty good, and I kinda wish I'd had a copy of it at the start of my university mathematical education, if only to teach me what the hell a "Lemma" was, exactly. It's also fun to read.
"21st Century Physics Flexbook" -- very odd. Admittedly intended as a K-12 textbook rather than a university-level textbook, but not much of a way to learn physics even for the kids. Virtually no mathematics whatsoever, let alone calculus. A random grab-bag of topics from "gravitation" (with an attempt to explain relativity which would be more confusing than enlightening) to "nanoscience" and "biomedical imaging". Basic mechanics finally comes in at the end as an afterthought. Not recommended to anyone.