Duke most wicked : a wallflowers vs. rogues novel
(Book)

Series numeration from fantasticfiction.com

"It is a truth universally acknowledged that a wicked duke who has gambled away his fortune must be in want of an heiress. Scarred by a dark secret, Brandan Delamar, Duke of Westbury, must concede his misdeeds have finally caught up with him. With five younger sisters to support, he must marry for money. Sunny and steadfast, Viola Beaton is no heiress. As music instructor to the duke's sisters, she's developed a genuine affection for the bright young ladies. Unfortunately, she's also developed a forbidden passion for her wildly attractive employer. It must be the way he inspires her to compose sonatas about moonlight and kisses. Or how his gaze smolders and lingers on her skin. Or because he makes her heart whisper impossible things. When Westbury decrees that he'll choose not only a bride, but grooms for his sisters, Viola can't allow him to curtail their freedom. She strikes a bargain: if he allows his sisters to attend the Season, Viola will chaperone them and keep them safe from scandal. Only...what if Viola and the duke are the ones most likely to cause a scandal?" -- Back cover.