Doyle's Gamble. Arthur Doyle, P.I. Series, #7

Doyle's investigating firm's business was in a slump with only cheating spouse cases. Then a man came in asking for help at his casino to catch someone who was cheating. He offered Doyle a lot of money to take the case, so Doyle accepted gladly. He organized the case with the help of his partner Drew, his secretary Marge and his girlfriend Poppy to go to the casino to watch the Blackjack tables where the most cheating was going on.
While on that case, Doyle was offered another to help protect a woman from Russian spies. She was being protected by the Witness Protection People but they had a leak and couldn't hide the girl until the trial without her being killed. Doyle and Drew staged a fake kidnapping of the girl and hid her at Poppy's apartment. Both cases where eventually completed but not without trouble for the witness.
This book is a novella, shorter than a novel, but longer than a short story. It's the 7th Doyle book of the series.