In my opinion, the best book in English on the background of Luther's Bible is J.M. Reu's Luther's German Bible: an historical presentation, together with a collection of sources (The Lutheran book concern, 1934). I've been looking for an affordable copy of this book for a long time. Used copies typically only go as low as $75. The price though can go much higher. I've used library copies of this book for years.
Good there is an online version. You can get the link by visiting James Swan's blog.
Beggars All: Reformation And Apologetics: J.M. Reu: Luther's German Bible... now online