Diamond of the Season by Tamara Gill

She was meant to find a husband—falling for her guardian was never part of the plan.

Lady Rosalind has one goal for her London Season—to secure a respectable husband who will provide her and her younger sisters with the stability their late father never did. As the new ward of the enigmatic and dangerously handsome Duke of Ravensmere, she should be focused on making an advantageous match. Instead, she finds herself drawn to the very man she cannot have.
Lord Nathaniel Blake never expected to inherit a dukedom, nor did he anticipate becoming the guardian of six young ladies thrust into society’s ruthless marriage mart. Bound by duty and honor, he knows Rosalind is off-limits. But when she steals a kiss beneath the shade of a willow tree, his carefully constructed restraint begins to unravel.
Determined to maintain propriety, Nathaniel tries to keep his distance, yet passion simmers between them. And when whispers of scandal arise—her father’s secret family, a conniving companion with designs on the duke, and rivals eager to see Rosalind ruined—she realizes that securing a match is the least of her worries.
With the stakes higher than ever, will Rosalind risk her reputation for the only man her heart desires? Or will duty and deception tear them apart before they have a chance at love?