The Bad Penny by Keri Brooks McWhorter

Lexi dared her boss and best friend Jake to ride down a steep hill. Result: one tumble, one dead horse, one nearly-dead Jake. The doctors think he won't make it through the night, so when Jake asks her to marry him in order to keep his greedy ex-wife from inheriting his ranch, she agrees.

Now they're stuck in this sham of a marriage, pretending to his parents and all the ranch hands that they're really in love. The worst part is, Lexi's falling for Jake, even though she knows she's no good for him; a bad penny that brings him nothing but trouble.

When his ex shows up claiming he's guilty of bigamy, Lexi has to choose between her one-sided love, and doing what's right.