Amazon has the Kindle edition of several Smithsonian books by DK Publishing on sale for $1.99 each:
Timelines of Everything: From woolly mammoths to world wars Kindle Edition
History Year by Year: The History of the World, from the Stone Age to the Digital Age
Explorers: Amazing Tales of the World's Greatest Adventurers
Maker Lab: 28 Super Cool Projects
Tech Lab (Maker Lab)
Our World in Pictures: Trees, Leaves, Flowers & Seeds:
Knowledge Encyclopedia History!: The Past as You've Never Seen it Before
When on Earth?: History as You've Never Seen It Before!
What's the Point of Math?
Timelines of Everything: From woolly mammoths to world wars Kindle Edition
History Year by Year: The History of the World, from the Stone Age to the Digital Age
Explorers: Amazing Tales of the World's Greatest Adventurers
Maker Lab: 28 Super Cool Projects
Tech Lab (Maker Lab)
Our World in Pictures: Trees, Leaves, Flowers & Seeds:
Knowledge Encyclopedia History!: The Past as You've Never Seen it Before
When on Earth?: History as You've Never Seen It Before!
What's the Point of Math?