Amazon has the DK Astronomy Book: Big Ideas Simply Explained (Kindle Edition eBook) for $1.99 downloaded. There are a number of other DK Big Idea books on sale for $1.99 as well:

• Math / Physics / Simply Psychology / Simply The Brain
• Literature / Shakespeare / Sherlock Holmes 
• Presidents Visual Encyclopedia / Big History / World War I / Military History
• Signs and Symbols / Africa: The Definitive Visual History