Her Horse Racing Billionaire Groom by Rachael Eliker

He has to be married by Christmas. She never expected to see him again. Will he be able to win her back?
Clark Kelly has a big problem: be married by Christmas or risk losing the charity he started with his father and his portion of the billion dollar family fortune. Up against seemingly insurmountable odds, Clark never was one to back down from a fight.
When Maren Davis applies for a job with a prestigious racehorse rehoming charity, she never expected to come face to face with Clark Kelly, the man who proposed to her years ago. And she really never imagined having all the old feelings come back. Good thing they are both professionals, bad thing he asked her to dinner.
Clark is convinced Maren is back in his life for a reason, but he has to figure out how to win her back. Being honest about his father’s will is an entirely different challenge. It doesn’t help matters when they run into an old boyfriend of hers, especially when that old boyfriend asks her on a date.
With the Christmas deadline looming, Clark has to prove he’ll do anything to win Maren’s love, even if it means sacrificing it all for her.