Boy, did I have a rough time whittling my list down to five (I was stuck at six for the longest!) but here, at last, are my picks. Check out Reformissionary to see what others recommend and add your own Big 5.
Questioning Evangelism by Randy Newman (I reviewed this one here)
Total Truth by Nancy Pearcey
Relationships: A Mess Worth Making by Tim Lane and Paul Tripp
Decision Making and the Will of God by Garry Friesen
Will Medicine Stop the Pain? by Elyse Fitzpatrick and Laura Hendrickson, M.D. (I gave a synopsis here)