I highly recommend the book "calming signals" by Turid Rugaas - has a lot of great illustrative pictures!
From my own files...
Rascal's a little hard to read IMO because his curly tail doesn't cue as much as, say, a Shepherd tail might. But his ears are quite expressive if you learn the nuances of his ear angles, and watching for tense muscles is also a dead giveaway.
Here's him a little uncomfortable but not at all frightened - note his ears are back a little bit (kind of angled parallel to his head, instead of sticking out perpendicular like, well, ears). His eyes are slightly closed and he's a little bit rigid. He's more wary/confused than upset at this point.
For contrast, here's him fairly stressed (by me being too close to him with the camera, and forcing him to hold still instead of run away) - notice the ear position, upright posture (almost leaning back away from me), and him licking his lips in a submissive gesture.
And again as comparison, this is him happy and relaxed - corners of the mouth soft and relaxed, kind of like a "smile," ears in a natural position (listening for something), body relaxed. He looks like he could easily be rolled over into a happy napping position, right? His eyes are still a little squinty because he's looking into the sun, though.
