Mirror Dance – Lois McMaster Bujold
This book takes some time out to look at the effects of multiple personality disorders.Mark (the clone I previously mentioned) is still slim enough to be able to impersonate Miles. This leads to the Dendarii Free Mercenaries (Miles’ private army & now ImpSec approved!) being led into a dangerous situation without Miles’ expertise.
Miles tries to save the day, and dies as a result. As we both know there are more books in this series, I don’t think I’d be spoiling your read too much to say Miles does return.