Michael Bird (Euangelion) blogs some references about the thought that Luke was the author of the Pastoral Epistles.

The main book to read (which is on my list but haven’t quite got there yet) is $amz(0281036764 S.G. Wilson’s Luke and the Pastorals). Witherington interacts with this one a bit, though Witherington is of the (much more reasonable, IMO) view that Luke is amanuensis, not post-Pauline author.

Check out Mike’s short bibliography.