Yahya Tawil over at All About Circuits has published an article on Arduino UNO’s electronic design and better understanding of its hardware:
This article explains how Arduino works from an electronic design perspective.
Most articles explain the software of Arduinos. However, understanding hardware design helps you to make the next step in the Arduino journey. A good grasp of the electronic design of your Arduino hardware will help you learn how to embed an Arduino in the design of a final product, including what to keep and what to omit from your original design.
Full details at All About Circuits homepage.