RuneScape Classic Wiki
RuneScape Classic Wiki
The Old School RuneScape Wiki also has an article on Book (Plague City).
The main RuneScape Wiki has an article on: Book (Plague City)

The book, titled Turnip Growing for Beginners, is obtained from Jethick in the Plague City quest. Jethick borrowed it from Ted Rehnison and asks the player to return it.

Attempting to read it results in the message Nothing interesting happens.