A large selection of eBooks from O'RilelyMedia/Make Magazine.
Looks like a must have if you are interested in getting into Raspberry pi or Arduino/electronics/hobby programming.
$0.01USD:
Make: Arduino Bots and Gadgets
Make: Making Simple Robots
Make: Getting Started with Adafruit FLORA
Make: Getting Started with Adafruit Trinket
MintDuino: Building an Arduino-compatible Breadboard Microcontroller
Make: Raspberry Pi and AVR Projects
Beat the Average:
Make: AVR Programming
Make: Getting Started with Raspberry Pi, 2nd Edition
Make: Getting Started with Arduino, 3rd Edition
Make: Getting Started with Sensors
Make: JavaScript Robotics
Make: The Maker’s Manual
MAKE 36: Boards and Microcontrollers
MAKE 38: High-Tech DIY
Make: A Raspberry Pi-Controlled Robot
Make: Basic Arduino Projects
Make: Getting Started with Intel Galileo
Pay $15 or more
Make: Sensors
Making Things Talk
nice spotting