That's what some of it is for. If Joker looks at the books, there's some on exploring omens and negative signs, some on the nature of faerie contracts, some on older traditions of magic. It's as if Strange picked clean the entire metaphysical section and stuck it on the table.

Still, Strange is a lying asshole who hopes that Joker won't look too closely at the titles.

"Indeed. Granted, none of these are as useful as the Circle's private collection, but sometimes one must deal with...unconventional sources. Inspiration happens in the oddest places."