Covert Fae by C.N. Crawford — book slip

In a post-apocalyptic world ravaged by angels, Ruby, a fae with the ability to glamour herself into any form, joins a secret organization known as the Institute.

Tasked with infiltrating the angels' court, she must pose as a succubus to gather intelligence and locate her missing sister.

Failure could lead to the world's destruction, compelling Ruby to harness her powers against the angels who devastated her world.

First in the 'A Spy Among the Fallen' series.