If you have just got an Intel Galileo you might find this book interesting. It tells you how to get started with the device and gives a good overview of the Arduino technology on which the Galileo is based.
To be honest, if you are experienced with the Arduino you will probably already know the majority of the content, but I reckon that there is enough Galileo specific content to make it worth the relatively modest asking price. It's also kind of useful to have all the content in one place. Each feature of the Galileo is well explained and there is also some useful background on the Intel aspects of the device.
Note that this book is written for a Galileo configured as an Arduino based platform and does not cover the use of the device as part of the Microsoft Internet of Things initiative.