如何开始硬件接口编程？

I have been doing desktop programming for a while but want to get started with interfacing with hardware. Specifically, I would like to learn how to use serial ports to take an external event and alert my application (so for example, I can turn on a camera when motion is detected by an external sensor). Please tell me how to get started, what type of sensors, what books (or online resources) are available. I tried Bing and Google but I need more pointers.I have been doing desktop programming for a whi