Luther did write prefaces to the Apocrypha. Though, after a quick search online, there doesn't seem to be a lot of resources readily available. That's a shame!
However, there is an outstanding study of the Apocrypha by Concordia Publishing House that I would highly recommend to anyone who's interested in the Apocrypha, Pseudepigrapha and the intertestamental period. It gives a lot of rich background leading up until the New Testament.
Even if you're not Lutheran, I think you'd find it quite agreeable and very educational!
The full title of the book is: The Apocrypha: The Lutheran Edition With Notes
It does contain excerpts from Luther's preface throughout.