6. The Mask of Dmitrios by Eric Ambler

(written Dec 3, 2015 catching up)
This is probably my least favourite of Ambler's early pre-war books.  It's still really good, but the set up never quite grabbed me, with the writer following the trail of Dimitrios across Europe. He isn't really in danger until the last third, so there isn't a lot of tension. The story and backstory of Dimitrios himself is really cool.  It also has one of the best descriptions of heroin addiction I've ever read.