I bought it just few days ago find it helpful but also i wish there was more context / explanation around some black magic syntax
If only to note, say, the section(s) of the Bash manpage or GNU Info Manual in which topics are discussed.
Taking the first-listed example, the '%%', '##', etc., substitutions are covered in Parameter Expansion.
I tend to go first to manpages, though I'm well aware that GNU deprecates these (exceedingly unwisely IMO) in favour of the Info Doc format (and its dedicated single-purpose reader). I'll note that the manpage alone renders to 120 pages (man bash | pr | less) onscreen, which is fairly formidable.
The PDF-formatted GNU bash manual (a fine format for a reference of this sort) runs 196 pages: <https://www.gnu.org/software/bash/manual/bash.pdf>
While there are exceptions, most packages have useful goodies in there to check first before one has to think about looking things up online.
What you'll find is both a set of pages & hierarchy of documentation as well as search (via swish++) through that space. And yes, there is an absolute trove of information available:
- Manpages
- Info pages
- Default package information
- Supplementary package documents, if installed
- HOWTOs from the Linux Documentation Project.
- Debian-specific documentation (installation, administration, security, policy, and other manuals)
- RFCs (divided into several groups, for manageability / relevance)
- Resources such as the Linux Gazette
There's also a commandline utility, dwww, which will invoke your browser of choice. Point that at a terminal-based browser (w3m, lynx, links, elinks, etc.) and you can quickly scan through docs from your terminal.