Dr. Aaron Schart of the the University of Duisburg-Essen put together an electronic bibliography on resources related to the study of the Book of the Twelve. Though the site appears not to have been updated since 2008, the list remains a valuable resource. The bibliography contains both monographs and articles on the text of the Twelve, interpretation, issues, ect. I have linked to it here.
One recent publication that is particularly noteworthy is The Thematic Unity of the Book of the Twelve by Jason T. LeCureux, a revision of his doctoral dissertation done under Gordon McConville and Pekka Pitkänen at the University of Gloucestershire. I just received this monograph and will make some comments in upcoming posts. Though not a cheap book, it looks promising as a helpful addition to the discussion of the unity of the Twelve. You can get it here.