Can be found at https://bitbucket.org/alfonse/gltut/downloads
There are PDFs and HTML format books in the archive file.
The license is mostly MIT with a small number of exceptions.
It looks like the it was deleted. Does anybody has these downloaded?
https://learnopengl.com/arcsynthesis.pdf
I think I've got the zip file with code and other stuff. I you need it I could upload them somewhere (I would need to verify the license).