Luther and the Scriptures by J.M. Reu. I just found this as a pdf, quite by accident. I don't know whose website put it up, but thank you.
In English, I don't think there's a better scholar on the construction and background of Luther's Bible than Reu. Many of his other books are available via Google and the Internet Archive, but this is the first time I've found this one.
His most extensive treatment of Luther's Bible was
Luther's German Bible: an historical presentation, together with a collection of sources. This is a book I've used, but don't have, and unless I'm mistaken, it's not either in print or on-line (if someone finds it online, please e-mail me). I've looked for a reasonably priced used copy off and on for probably 10 years as well. Most copies are usually between $75-$150.
Update: The link no longer works. I managed to save a pdf of this book, so if someone really wants a copy....