Mathematics for Computer Science (2017) [pdf]

A more recent version of this set of notes is available from the MIT course page: https://courses.csail.mit.edu/6.042/spring18/mcs.pdf

It's worth reading at least the first couple pages where they contrast the meaning of truth, or "proof" across math, philosophy, a courtroom, a business, etc.

I heard axiomatic proofs are not always taught in US school geometry (typically 9th grade?) anymore.

If that's true, I wonder what is the first educational exposure supposed to be nowadays to this kind of thinking.

Another great book that fills a similar purpose is Volume 1 of the Art of Computer Programming by Donald Knuth. It presents a great introduction to (basic) math as computer scientists use it.

the site looks very awesome.

  Open Data Structures covers the implementation and analysis of data structures for sequences (lists), queues, priority queues, unordered dictionaries, ordered dictionaries, and graphs.