Her Heart's Desire by Ruth Ann Nordin
As soon as Sally Larson meets Rick Johnson, she knows he's the one for her. The catch? He's interested in her friend, Ethel Mae. But Sally has a plan. With Ethel Mae's help, she convinces Rick that the only way he'll get Ethel Mae is to make her jealous. He agrees, but he has a trick or two up his sleeve. Sally just might find that the one playing cat is really the mouse.