proc.txt in Linux souce code on github show the meaning of btime of the file /proc/stat in chapter 1.8 Miscellaneous kernel statistics in /proc/stat.
The "btime" line gives the time at which the system booted, in seconds since
the Unix epoch.
Some helpful books are included in the Guides of The Linux Documentation Project.
Linux Filesystem Hierarchy outlines the set of requirements and guidelines for file and directory placement under the Linux operating system.
