The Earl's Engagement (Reluctant Grooms #5)
Anne Gallagher

After the lady in question marries her true love, Rory is left unoccupied. Seeking out an old acquaintance, he discovers Lord Briden nearly penniless, mentally unstable, and entirely dependant upon his spinster daughter for the care and upkeep of their estate at Primrose.
The beautiful Lady Rosamund is part hoyden, part bluestocking, part servant. Wanting nothing to do with Society, her uncommon way of life spurs Rory into offering assistance, but Rosamund adamantly refuses. When Rory learns the lecherous lord next door is blackmailing Rosamund, he devises a scheme to help her whether she wants it or not.
The idea to find Rosamund a husband goes slightly awry, as Rory doesn’t bargain for falling in love with the enigmatic Lady Rosamund. And when his plan turns up another beau for Rosamund, can Rory let her go?