Check out the Ospery Home page, especially the WW I A/c of the Aces series , prices are down to $12.95 so would be a good time to pick up any you are missing. The paperback of the Pup Aces is sold out and only available as an e-book.
You'll recall that some months back there was talk of Ospery not doing any more WWI books , and they may not reprint those already done.
Ed