I've just read BUTB too and I agree that it was brilliant. I know what you mean about not wanting such an experience to come to an end. For me, I usually don't try to read something equally memorable immediately .. I might read a good crime novel and then return to a more literary book.
Last week, however, I followed BUTB with My Dear I wanted to tell you, set in WW1 .. a beautifully written love story (not chick-lit though, as I said in another post) but also dealing with the horrors of trench warfare and the brutal beginnings of reconstructive surgery in that era.
If you want something with an absorbing historical context maybe try Restoration by Rose Tremain? I'm looking forward to the sequel soon - it's been a long time coming!
Or Sacred Hunger by Barry Unsworth. The story of a slave ship. It made a huge impression on me the first time I read it; just about to re-read.
Ann Patchett is an amazing writer .. Bel Canto would be my favourite novel of hers but State of Wonder is also excellent. Very intelligent and the sort of books that stay with you for a long time.
The Observations by Jane Harris is an entertaining read; the narrative voice of a young Scottish servant girl is very cleverly done.
Of course the third part of HM's trilogy will be along in a couple of years :)