I like Jane Austen, especially Emma and Pride and Prejudice. Her portrayal of human nature still resonates today and she doesn't shy away from contraversial topics. Her oddly distant narration is something that isn't common in modern prose (at least not the stuff that I've read) but it works quite well.
My very favorite "grown up" books are The Beekeeper's Apprentice and it's sequels (A Monsterous Regiment of Women, A Letter of Mary, The Moor and O Jerusalem) by Laurie R. King. These mysteries pair up an aging Sherlock Holmes with young, half-American, feminist Mary Russell in the early decades of the twentieth century. The books are exquisitely written and have great characters. I'd recommend them to anyone except for die-hard Sherlockians who are deathly opposed to character development.
Another series that I enjoy is Elizabeth Peters' Amelia Peabody books (the first is Crocodile on the Sandbank). These hilarious romantic mysteries follow Amelia, a forward thinking Victorian gentlewoman, through her adventures in detecting, Egyptology, matchmaking, marriage, and motherhood.
Peters also writes a wonderful series featuring modern day art historian/amateur sleuth Victoria Bliss. The banter between Vicky and her sometime nemesis, sometime lover (art swindler Sir John Smythe) is positively priceless. The Recorded Books audio versions of these books are terrific also. Vicky's first adventure is chronicled in Borrower of the Night. Her first encounter with 'Sir' John is in Street of the Five Moons.