Do you have unread books on your shelves? I have just read a book that must have sat there all of 30 years unread. Anyway I finally got round to the little paperback The teaching of the epistle to the Hebrews by Geerhardus Vos. It came out in 1956 and contains five chapters of lecture notes tidied up by Vos's son Johannes. I have long been aware that Vos takes the view that (if we may out it this way) the Hebrews were Gentiles. The argument seems to make sense but it is hard to swallow. On other matters he is less surprising but always very thorough an careful, asking awkward questions and pursuing them until he has satisfactory answers. Hebrews is very different to any of Paul's letters and Vos helpfully points out not only where that is so but suggests why too. This 125 page book is an obvious read for anyone working their way through the epistle. The book has a new cover these days. Vos also wrote two or three essays on the same subject for the Princeton Review. These are online here, here and here.
Also see this interesting article on Vos's writings.