Don't mention it! Plus, I read Straub's 'Mystery' several weeks ago and, very delicately, it's also connected with the events described in 'Koko' (Underhill's book is mentioned as slightly distorting the actual murders), and I believe the next book, 'The Throat', completes the trilogy in a way. But I don't know if 'The Throat' EXPLAINS anything from 'Koko' as I haven't read this one yet.

'Mystery' itself is a very compelling read but don't expect 'Koko 2': it's more like an homage to Poe, Conan Doyle and Mark Twain than come back to the Vietnam battlefield. It's not as violent or dark as 'Koko' either, but it holds more surprises for you: I agree that the killer identity was predictable in 'Koko' (I felt exactly as you did: 'What is this Straub thinking? This is supposed to be the final twist???'), and here it's, well, exactly the same, but there are some other things which will make you scream 'How could I not see THAT?' - which is always nice. Have gripping reads!