Would it be possible to put a whole book (public domain) into the game? I have it in the form of a text file. Do you think it’s feasible? Where you could click on each chapter and read it?
It is.
AGS has File functions that let you read any file. Unfortunately it still does not support moving these files into the package, so it has to be placed by hand along with the game.
File functions reference:
https://adventuregamestudio.github.io/ags-manual/File.html
Alternatively, you may copy whole text into the game script, and store it as an array of Strings, for example.