A Knowing One — Judith Hale Everett

Just when Tom Breckinridge has decided to propose marriage to Diana Marshall, a rival has come out of the woodwork: Reginald Popplewell, a childhood neighbor of Diana's, and now heir to a viscountcy. Diana's father much prefers Reginald, but Tom believes Reginald has something to hide. When Mrs. Marshall invites Tom to a house party at their home near Brighton, Tom finds even more to dislike about dear Reggie, but Diana won't hear a word against him and goes so far as to wring a pledge from Tom that he will make a friend of Reginald. Knowing just how risky it can be to befriend a con man, Tom will discover to what lengths he is willing to go to win Diana.