Have a look at http://www.gutenberg.org/wiki/Scienc...m.2C_1904-1964
There are two series. "Terro-Human Future History" and "Paratime". Best do both as an omnibus. It would not hurt to make a third omnibus of the other SF from Piper.
"Murder in the Gunroom" should be a separate book. Maybe "A Catalog of Early Pennsylvania and other Firearms and Edged Weapons at 'Restless Oaks' McElhattan, PA" can be added, because "Murder in the Gunroom" is about a gun collection.
"Rebel Raider" is a biography/report from the US Civil War. Not really in popular demand.