"I need you to owe me something," Patricia said, "or this won't work. I'm really sorry. I tried to do it every other way, and none of them succeeded. In the end, the most powerful magic is often transactional
in some way."
Patricia to Laurence in All The Birds in the Sky by Charlie Jane Anders.
Other adult books that feature magic:
Barker, Emily Croy. The Thinking Woman's Guide to Real Magic.
Flyte, Magnus. City of Dark Magic.
Grossman, Lev. The Magicians.
McGuire, Seanan. Every Heart a Doorway.
Schwab, Victoria. A Darker Shade of Magic.
Walton, Jo. Among Others.
Young adult books that feature magic:
Marillier, Juliet. Wildwood Dancing.
Bow, Erin. Plain Kate.
Black, Holly. The Darkest Part of the Forest.
Durst, Sarah Beth. Ice.