My French Professors in France always demanded rigourous proof from us, but never emphasized elegance is equally important, if not more.

I like the short and elegant proofs in the side-bar window-box, e.g. Lagrange Theorem on subgroup, etc.

To say it to be understood by High School or undergraduate students is a bit exagerated. Frankly, some of the proofs are quite complex and abstract, I think they are at Masters level.

I recommend it to be in any library or home bookshelf.