Making Things Talk focuses on building various devices with the Arduino microcontroller board. The Arduino is a very cool little board with digital and analog I/O ports, a USB port that you can also use to power the board (or you can use the wall-wart and built in regular power plug). Programming the Arduino is very easy since the language and programming environment is based on Processing, a stripped down Java language that hides all the complexity of dealing with hardware ports, graphics, and much else. If you want to get into making robots or physical computing but are intimidated by the learning curve of most microcontrollers, Arduino is for you. The project website Arduino.cc has a lot of information, there are tons of Instructables involving Arduino, and the Make magazine blog has an Arduino project every couple of days it seems like. There are also a bunch of different "models" of Arduino, ranging from very small ones that connect to a breadboard to the Lilypad model that you can sew into clothes and even wash!
As for the book, I'm very impressed so far. It doesn't mess around with a bunch of little projects that you get bored with or skip like a lot of other books. The first project is a Pong game on a computer using a monkey doll with flex sensors in the arms to control the paddles. From there you move onto using the Bluetooth addon for the Arduino to make the game wireless. It's enough to challenge you but not hard enough to get discouraging (unless you count being discouraged because you have a limited budget for this type of thing, the Arduino and bluetooth addon are over $100). The descriptions of what you need to do are great and there are tons of pictures.
I'm going to be diving further into the book soon and I'll update the review as I progress but if you've ever wanted to get into physical computing, I'd strongly recommend the Arduino and Making Things Talk.