The black sheep and the hidden beauty / Donna Kauffman.

Enhanced descriptions from Syndetics:

They're back--the boys you go out looking for precisely because your mother warned you not to--the bad boys every good girl needs at least once, if not twice. . .

Raphael "Rafe" Santiago may have left the streets years ago, but the street has never left him. A rough childhood in the Bronx taught him never to let his guard down, to keep everything in order, and always to trust that little voice in his gut that tells him when someone's got something to hide. horse trainer Elena Caulfield, is definitely hiding something, and Rafe intends to find out what it is and take care of it--his way.

But his way wasn't supposed to include feeling an intense attraction to the tomboyish Elena. With her mud-caked boots, quiet strength, and gentle manner, she's nothing like the flashy, seductive, overtly feminine women Rafe usually beds. The closer he gets to her, the harder it is to control that fiery passion he's worked hard to keep cooled, the kind that can catch a man off guard and leave him open to danger--because whatever secret Elena's protecting, it's big. . .and worth killing for. Because when you're from the Bronx, you take care of what you love--or die trying. . .