Tom Igoe have written two books:
"Physical computing" that explains how to hook just about anything up to a microprocessor. This book is very good and explains in detail all the circuits in it. The mircoprocessors in the book are not Arduino, but the code examples and circuits in the book can relatively easyli be used with Arduino.
"Making things talk" is Tom Igoes book about a whole lot of projects based on Arduino that in some way commnicates soem kind of information, including over the internet.
I really recomend both these books.