One Lucky Cowboy by Carolyn Brown (pdf, epub, mobi)

Jane Day is on the run from the paid assassin who had been her fiancé. In Wichita Falls, Texas, she meets Nellie Luckadeau, a spitfire of an old lady who desperately needs someone to work on her ranch. But Nellie's drop-dead gorgeous grandson "Lucky Slade" is sure he can spot a con artist a mile away. He's determined not to let some upstart like Jane fleece his granny.

When his signature intimidation methods don't convince Jane to leave, he pours on the charm to make her spill what she's up to. She's happy to play along, but she's not going to let this hot, hostile cowboy run her off his land when all she needs is a lucky break…