This Is Murder by Erle Stanley Gardner (WDL, 1960)

Sam Moraine, businessman, poker player and friend of the D.A. agreed as a lark to act as go-between in a kidnapping case.  Following instructions, he went to the boat, delivered $10,000 in old bills and received in exchange a bundle of blonde dynamite named Ann Hartwell.

As they reached the mooring float, they were met by the beam of a flashlight and a man's voice saying:

"You're under arrest, both of you.  Get your hands up in the air and keep them that way."

It was the Federal boys, and when they'd finished working him over, Sam thought the case was finished.

But it was only beginning.  What had started out as a simple adventure turned into a boiling mess of crooked politicians, double-crossing cops, ex-cons, a certain lady of elegant if easy virtue - and MURDER!

 Of course it did.

This is one of Gardner's earlier novels (first published in 1935 under the name of Charles J. Kenny) and not part of any of his series.   He'd been writing for nearly fifteen years before that, however, and he'd honed his craft publishing in magazines such as Black Mask and Detective Fiction Weekly.  In other words, he was already a slick and professional crime writer by the time this entertaining yarn was first released.