My sister is obsessed with the Wimsey/Harriet Vane books (at first because her name is Harriet, too), but I've never really cared for them. On the other hand, I love Jill Paton Walsh. She's really quite extraordinary. My favorite book of hers is "The Emperor's Winding Sheet," about the last emperor of Constantinople when it was under siege by the Turks. It's technically classified as a YA novel, but there's a lot of depth to it and impressive historical detail. After reading it, I felt like I'd been there. I couldn't put it down.