I did Art of Problem Solving in high school which has excellent textbooks. Certainly not for everyone though.

There are many many wonderful math books, and many many wonderful math resources on youtube. (As well as, of course, many many terrible ones!) I second AOPS! They have a fantastic series for younger kids called "Beast Academy" (https://beastacademy.com), and their challenge problem sets are fantastic! The same blog has a couple of other posts about them: https://leosstemhacks.wordpress.com/2019/11/06/the-three-bes...

There's also Life of Fred which is definitely different to anything else out there.

Israel Gelfand wrote a number of amazing books covering many areas of high school math.

I enjoyed Measurement [0]. I was unsurprised when I eventually learnt that its author had previously written Lockhart’s Lament [1].

[0] https://www.hup.harvard.edu/catalog.php?content=reviews&isbn...

[1] https://www.maa.org/external_archive/devlin/LockhartsLament....

The Singapore Math curriculum is fantastic.