Staying Dead by Laura Anne Gilman

Rediscover the world of the Retrievers and the Cosa Nostradamus, in book 1 of the popular series by Laura Anne Gilman.

It starts as a simple job – but simple jobs, when you're dealing with the magical world, often end up anything but. As a Retriever, Wren Valere specialises in finding things gone missing – and then bringing them back, no questions asked. Normally her job is stimulating, challenging and only a little bit dangerous. But every once in a while…
 
Case in point: A cornerstone containing a spell is stolen and there's a magical complication. (Isn't there always?) Wren's unique abilities aren't enough to lay this particular case to rest, so she turns to some friends: a demon (minor), a mage who has lost his mind, and a few others, including Sergei, her business partner (and maybe a bit more?).

Sometimes what a woman has to do to get the job done is enough to give even Wren nightmares…