The GNU project has an official manual freely available on their website.
That said, it seems like a Herculean task (and a waste of time to a certain degree) reading it entirely in one go since the number of tools in Coreutils is too vast, and some of them do not see much use except for fairly specialized tasks.
Thus, you will likely be better off look up the topic or the command of your interest as supplementary information for what you've read in the man pages and Internet forums.