I'm reading Legend now. It holds my interest, but it's nowhere near as well written as THG. The main characters are supposed to be 15, but they are presented with a maturity level and presence of people more likely in their late 20s to 30s.
As for Cornwell, there is no comparison of it to Legend. They are far too different. If you like her books, then you should try the series by Kathy Reichs whose main character is a forensic anthropologist (the author IS a forensic anthropologist so she does know what she's talking about). I think the first in the series was Deja Dead. Another series that uses some medical knowledge is by Tess Gerritson. Her first book was The Surgeon. As for detective crime solving mysteries, I absolutely LOVE J.D. Robb's In Death series. The first is Naked in Death. If you want funny crime solving, try Evanovich's series about Stephanie Plum - the first is One for the Money.