A book on the Contiki OS and programming for Contiki is available: “IoT in 5 days: an easy guide to Wireless Sensor Networks (WSN), IPv6 and the Internet of Things (IoT)” by Antonio Linan Colina and Alvaro Vives and Antoine Bagula and Marco Zennaro and Ermanno Pietrose. You can download the last version of the book here, or you can visit the home page of the book here.