And William Le Queux as well.
http://en.wikipedia.org/wiki/William_Le_Queux
Not a lot of his books are actually ebooks for some reason. But Many Books has some
http://www.manybooks.net/authors/queuxwil.html
Seems most of his spy stuff was written around WW1. Later it was more mystery, earlier more adventure. Not as good as Oppenheim...