Remember - you don't necessarily need to read an entire book to fill each square. The two collections are both chock full of "locked room" mysteries and other impossible crimes. The Black Lizard edition, edited by Otto Penzler, is a long book (900+ pages), and the Martin Edwards offering is part of the British Library Crime Classics series. One or two stories would fill this square nicely!
In addition, Murder on the Orient Express is such a wonderful book that everyone should read it! There is also a new adaptation coming soon to a theater near you, so if you plan to see the movie, always read the book first!
Finally, Death in the Tunnel has received a mixed reception here on BL. Both Tigus & I really liked it, but BrokenTune (I think) was decidedly lukewarm.