Simon Brett writes great mysteries becuase the parts that don't deal with the mystery could easily make a good book on their own. I think these are the best sorts of mysteries, just like, say, Singin' in the Rain is a great movie becuase it's great even without all the singing and dancing. Sicken and So Die does an excellent job as a novel about the theatre, but the mystery is just as important, and it's not that well done this time.
In Sicken and So Die, Charles Paris, the weary middle aged actor, is working in Twelfth Night and is having a nice time doing it. Then, the director is poisoned and a new avant-garde one takes the helm, much to Charles' annoyance. As he valiantly continues to play his part the way he thinks it should be done, he investigates a series of accidents that have happened in the play. Could they not be accidents?