The Wrong Familiar by Megan Derr

Ninos has been training all his life to be a sylvan mage. Trees, plants, herbs, flowers, fruits, berries, nuts—he knows them all, and he intends to use that knowledge for the good of his home, a village he has not seen in more than a decade.
The final step to becoming a full-fledged mage is the conjuring and binding of a familiar, a faithful companion and guide who will supplement his power and remain at his side the rest of his life. Instead of a sylvan familiar, like a unicorn or stag, however, something goes horribly awry and Ninos finds himself bound instead to Sinn, a raven familiar meant for dark magics.
Neither is happy about it. Ninos wants a familiar who can help him with life, with growing and restoring a place destroyed by fire and plague. Sinn should be helping to raise the dead or cast blood workings. They were never meant to meet, let alone be bound.