Similar books: Lord Mouse, Thick as Thieves

This book is a light humorous adventure/quest. 

Brandsome the Unicorn lost his horn. He hires a PI named Twig, half-dragon/half-elf, who takes the job and promises to find the horn in question and bring it back. 

Being put down as a collateral for down payment, Quinn, the unicorn's human sex slave, takes on the role of a side-kick to Twig.

After three years of slavery and abuse, Quinn is happy for a short break, even more so because he adores his handsome new (if temporary) master. Of course, there are flashbacks and discussions of slavery under unicorn's hoof, but Quinn is living in the moment and enjoying his short freedom, not wanting to dwell on his misery. Twig treats him like an equal, at the same time being a bit on over-protective side due to his dragon-half mating surges. Twig Starfig is tough and rough, but knows firsthand how the rest of the world treats those who don't fit in it perfectly. 

The odd couple - a plain non-magical human pet and a half-breed - travels around the realm, playing dangerous card games with elves, boarding ghost ships, fighting zombies, drinking tea with cyclopes and making acquaintances with demons in penis-shaped castles (yep, that's a thing!). 

There is a bit of angst in the end, seeing how Quinn can't go back to Brandsome-the-Unicorn, not if Twig has any say in it, but mostly this book is about adventure and how it brings two odd creatures together. Love conquers all, they say =) Well, that and a very influential father-elf ;)