At this time, complete texts of seven of the eight Doctor Syn novels are available here.

Doctor Syn on the High Seas

Doctor Syn Returns

The Further Adventures of Doctor Syn

The Courageous Exploits of Doctor Syn

The Amazing Quest of Doctor Syn

The Shadow of Doctor Syn

Doctor Syn

The Scarecrow Rides
(an extended version of Doctor Syn Returns)

All texts courtesy of the late, except for The Scarecrow Rides, which was prepared by Jordanna Morgan by courtesy of Will Curtis.