A Date with Dishonor (Harlequin Historical)

is book #1157 in the Harlequin Historical line.
==Book Description==
Lady seeks kind gentleman to offer protection… 
When a mysterious lady advertises her charms in the newspaper, there's no way Viscount Blackthorne will allow his rash friend to attend the twilight rendezvous. Taking his place, Blackthorne is surprised by the reluctant beauty who appears—she's far from the scheming courtesan he was expecting. 
Elise Dewey must protect her foolish sister by posing as "Lady Lonesome" in her stead. She's shockingly stirred by the imposing stranger who waits for her in Vauxhall Gardens—but their liaison has been observed… Unless Elise accepts the viscount's bold proposal of marriage, they will all be plunged into scandal!
(Description from harlequin.com)