Getting Started with Arduino
If you are fluent with Xcode and C/C++ programming but you discover the Arduino boards and the embedded computing world, I recommend the book Getting Started with Arduino by Massimo Banzi as a good first step-by-step approach.
This book addresses the very basics and answers all the questions a starter can ask about embedded computing. However, it is pretty limited and it doesn't go beyond blinking a LED.
I strongly suggest to read this book before purchasing the first board, in order to have an idea of the many fields implied by embedded computing.