Right then. Let's spend ten minutes on this.
Message: Count off every fifteenth book starting on the third one, putting your finger on each such book so that I can see the title each time. Then, open to three random places in each such book, and show each such random place to the scry sensor for six seconds. Then buy them all. You can haggle that time.
Keltham was initially thinking of just buying books that discussed magic or gods or afterlives, then considered that his fortress doesn't have enough books anyways.
If their mystery genre is at all like dath ilan's, if there's even a trace of sanity there, key obscure rules of magic should feature in at least some of these novels; and you can't rewrite fiction as easily as nonfiction.