Search - The Best Intentions (Signet Regency Romance)

From back of book:

After the death of his beloved wife, the dashing Earl of Strickland has finally decided to seek out a new mother for his children. But he'll consider no giddy, novel-reading young girls with stars in their eyes.
Resigned to having lost his one true love, he is also resigned to a practical, dispassionate, respectable match with a mature woman.
That is, until a very unexpected - and very special - houseguest throws his careful plan awry.

Hannah Fairbanks would gladly leave the marrying - and remarrying - to her beautiful and refined older sister. Hannah herself would much prefer to tramp about the countryside, muddied hems and all, studying the ancient ruins and architecture that are her one and only true passion... or so she is convinced, until a fateful visit to an earl' estate - and an even more fateful kiss - throws everyone's plans awry.

A delightfully witty classic regency!