1. His free course: https://products.easyperf.net/perf-ninja
2. His free book: https://book.easyperf.net/perf_book (the 2nd edition is being worked on right now and there's a draft on github: https://github.com/dendibakh/perf-book)
Of course, it doesn’t cover every possible performance trick. For that, I’d also recommend the Intel Optimization Manual and Johnny’s Software Lab.
Funny, I've just discovered them via a totally different route and read a few articles. Definitely good resource to learn more about low-level CPU code optimization.
Though OP doesn't say what are they interested in. GPU code optimization, for example, is totally different bonkers Universe :D