https://cdn.kernel.org/pub/linux/kernel/people/paulmck/perfbook/perfbook.html
https://mirrors.edge.kernel.org/pub/linux/kernel/people/paulmck/perfbook/perfbook-e2.pdf (reading version v2021.03.21a)
https://git.kernel.org/pub/scm/linux/kernel/git/paulmck/perfbook.git
New releases should appear three to four times a year, give or take.