If I'm not mistaken, this is written by the author of Lucia, a popular auth library for TypeScript [0]. He recently announced that he will be deprecating the library and be replacing it with a series of written guides [1], as he no longer feels that the Lucia library is an ergonomic way of implementing auth. He posted an early preview of the written guide [2] which I found enjoyable to read and complements The Copenhagen Book nicely.
[0] https://github.com/lucia-auth/lucia
[1] https://github.com/lucia-auth/lucia/discussions/1707
[2] https://lucia-next.pages.dev/