Fans of free youth adventure e-books will quickly encounter Robert Michael Ballantyne. There are a LOT of Ballantyne titles available at Gutenberg; over 100! I've tried to organize the Ballantyne catalog, grouping similar titles in bundles of five or six. I haven't read all of these, of course, but assigned categories based on a quick perusal of each, which is, I admit, a very subjective process. The categories certainly overlap; a pirate story is by definition also a nautical story. Some titles that weren't actually fiction, or that didn't fit any category, were omitted.
At any rate, here is my attempt. Each category might make a nice little omnibus e-book, don't you think?
Stories of Africa
• Blue Lights
• The Fugitives
• The Gorilla Hunters
• Hunting the Lions
• The Settler and the Savage
• Six Months at the Cape
Exploring The Arctic
• Chasing the Sun
• Fast in the Ice
• The Giant of the North
• Red Rooney
• The Walrus Hunters
• The World of Ice
The Canadian Wilds
• Away in the Wilderness
• The Big Otter
• Fort Desolation
• The Pioneers
• Ungava
• Wrecked but not Ruined
Dangerous Work
• Deep Down
• Fighting the Flames
• Life in the Red Brigade
• The Lively Poll
• The Young Trawler
Exploration (a bit of a catch-all)
• Lost in the Forest
• Martin Rattler
• Rivers of Ice
• The Rover of the Andes
• Under the Waves
• Up in the Clouds
Historical Fiction
• The Cannibal Islands
• Erling the Bold
• The Hot Swamp
• Hunted and Harried
• The Norsemen in the West
Maritime Tales (lifeboats, lighthouses, steamships, etc.)
• Battles with the Sea
• The Floating Light of the Goodwin Sands
• In the Track of the Troops
• Jeff Benson
• Saved by the Lifeboat
• The Story of the Rock
Exploring Native America
• The Buffalo Runners
• Over the Rocky Mountains
• The Prairie Chief
• The Red Man's Revenge
• Silver Lake
• Twice Bought
Nautical Yarns (age of sail)
• The Battle and the Breeze
• Black Ivory
• The Crew of the Water Wagtail
• Fighting the Whales
• The Lifeboat
• Sunk at Sea
Pirate Tales (ARRRRR!!!!!)
• The Battery and the Boiler
• The Coral Island
• The Madman and the Pirate
• The Middy and the Moors
• The Pirate City
Sailing The South Seas
• Blown to Bits
• Gascoyne, The Sandal-Wood Trader
• The Island Queen
• Jarwin and Cuffy
• The Lonely Island
• The Red Eric
Western Adventures
• Charlie to the Rescue
• Digging for Gold
• The Dog Crusoe and His Master
• The Golden Dream
• Philosopher Jack
• The Wild Man of the West