I have recently added the following titles to the electronic books listing:
Thomas Aquinas, Commentary on the Epistle to the Hebrews.
Ian Pennicook, The Shadow and the Substance: A Commentary on the Letter to the Hebrews.
Ray C. Stedman, How to Live What You Believe: A Life Related Study in Hebrews.
The last two are more popularized expositions on Hebrews.
Update: I have added about a dozen more titles including those by W. H. Boulton, Friedrich Buchsel, Martin Dibelius, Emmanuel Hirsch, Julius Kogel, Alexander Nairne, Paul Padva, Eduard Riehm, David Schultz, Alfred Seeberg, J. G. Simpson, and Karl Wieseler.