Commit 215ee8a0a975323737d0d5682f71e18321b07ba8 moved the book definitions to JSON.
However, it seems to me that the JSON definitions for the books are not used by the game yet.
Anyways, here is the JSON snippet for “The Art of Rhetoric”:
{
"type" : "BOOK",
"id" : "manual_rhetoric",
"name" : "The Art of Rhetoric",
"max_level" : 6,
"description" : "A translation of an ancient yet still comprehenshive text about speech.",
"weight" : 2,
"to_hit" : 1,
"color" : "blue",
"intelligence" : 10,
"symbol" : "?",
"material" : ["PAPER", "NULL"],
"volume" : 4,
"bashing" : 8,
"cutting" : 0,
"time" : 30,
"fun" : 0,
"skill" : "speech",
"rarity": 20,
"price": 200,
"required_level" : 0
}
As I said, I haven’t been able to test this book ingame, so do not consider above versioion “final”. 
Question: What are the color conventions for books? Especially, Does the color blue mean “advanced readers only” or does it mean that it covers advanced topics but makes no assumptions on wheather it may also be read by beginners? I assumed the latter. My book goes from 0 to 6, so would it be a good idea to introduce a new color?