First: if you're near a hackspace (and here's the big list), go there: you'll learn far more from playing with other hardware hackers than you will on your own!
Weblinks:
- publiclab.org - Go check out the PublicLaboratory community - they have lots of cool projects, all designed for creation and use anywhere
- sensornews.org - Sara's notes on using sensors in journalism
Books:
- Charles Bell, "Beginning Sensor Networks with Arduino and Raspberry Pi" - just go build it!
- Emily Gertz, Patrick Di Justo "Sensor Networks" - on building environmental sensor networks