I remember these stories with great affection, they were in my father's collection of books from his youth and I read them over and over. This is the first story in Mr. Snell's
Mystery Series for Boys
1. Triple Spies (this ebook)
2. Lost in the Air (available
http://www.gutenberg.org/ebooks/10599)
3. Panther Eye (available
https://www.mobileread.com/forums/showthread.php?t=47787)
4. The Crimson Flash (available
http://books.google.com/books?id=udQ...page&q&f=false)
5. White Fire
6. The Black Schooner
7. The Hidden Trail
8. The Firebug
9. The Red Lure
He also wrote;
The Radio-Phone Boys’ Series
1. Curlie Carson Listens In
2. On the Yukon Trail
3. The Desert Patrol
4. The Seagoing Tank
5. The Flying Sub
6. Dark Treasure
and
Adventure Stories for Girls
1. The Blue Envelope
2. The Cruise of the O’Moo
3. The Secret Mark
4. The Purple Flame
5. The Crimson Thread
6. The Silent Alarm
A few are available at
http://www.gutenberg.org/ebooks/sear...ads&query=3593
and some more are available at
http://www.google.com/search?tbm=bks...3fbfbda4b88cf3