Other than a few commercial third party guides for iPad iOS, no apparent
Apple Books successor had arisen to that stature, so far; unless recently.
iPad User Guide (Table of Contents) - Apple Support
//support.apple.com/guide/ipad/toc
Read and bookmark the iPad User Guide - Apple Support
An option to use the Table of Contents page and modern browsers, to
bookmark for easier access; & use that instead of general front pages
could help. These TOC's from Apple are good guides, for iPhone, iPad
and various Mac models. ~ Earlier versions online guides, used links to
keep their version contents intact; a better longterm owner reference.
These can be saved; almost like PDFs, in Preview for use offline. However
generally one may use the web pages; to find later changes to your model
device were superseded, by a guide-version no longer in your possession.
iPadOS 17 - Apple
"See the full list of what’s new
in iPadOS. Learn more (PDF)."
Older version link to online resource may temporarily somehow get skipped
and not superseded by a newer iOS version guide; and remain useful longer.
..As the hardware guides for system use become outdated as those change..