It's random :)
Since all the books are books added by you which presumably you own, I would assume a random suggestion would suffice :D
The tool doesn't use the internet in any way don't worry ^^
Each time you open the app, if there are books you haven't read, or if there's at least a series unfinished from the list, you will receive a window with a recommendation :D
The system works best if you have a bunch of unread books, but you can always turn it off in preferences if it bothers you!
Have a nice day ^^
I got stuck at one point because I couldn't work out which item to use on which object, and the game didn't really explain anything after I did something. I liked the idea, but consider making the consequences of the player's actions more apparent. The game was good overall, very nice visual design which to match the theme!
Hope you had fun making it!! ^^