For those who really enjoy the "It's him, no it's her, no it's him...oh crap, I give up!" aspect of LOCI and how you will sometimes have six suspects (or it feels like it), I've got a great recommendation for you.
Rendezvous, by Alastair MacNeill. It's a WWII thriller set around trying to find a traitor on an SOE mission and trust me, it will keep you guessing. Or as someone famously said in the Proud Flesh thread, you'll think you did it by the end of it:)
I would recommend "Blood and Money" by Thomas Thompson. I read this book a couple of times, and felt that way. Did he or didn't he, with everyone in the book as to what part they played in it. Great read. Sad to say, its based on true events
“It’s like you got yesterday, today and tomorrow, all in the same room. There’s no telling, what can happen”.... Billy, "I’m Not There"