Just one link to the Macmillan page since it has links to B&N, Google, Amazon, and Kobo. All these are $2.99 apiece.
Old Man’s War John Scalzi
Dragon Mage Andre Norton & Jean Rabe
Beast Master’s Planet Andre Norton & Lyn McConchie
Beast Master’s Quest Andre Norton & Lyn McConchie
http://macmillanedeals.com/#sff