A Darker Shade of Magic (Shades of Magic, #1)

by V.E. Schwab

The only items on display were a book of poems, a glass ball filled with black sand, and a set of maps. The poems were by a man named Blake, and had been given to Kell by a Collector in Grey London the year before, the spine already worn to nothing.

Throughout the series there are a few things that tie Kell and Lila's fates together. This is one. In A Gathering of Shadows we learn that Lila's mother read her Blake poems, and consequently Lila uses them to help her summon and focus magic.