Mike Caro (a.k.a. the Mad Genius) wrote an excellent book on reading players. The book applies mostly to real-life games, but it is well worth a read, even for online players. For those playing live, whether it be in a home or a casino, the book is full of photos of players demonstrating different behaviors. The clothing styles date the book, but the cast of characters is timeless and apply today as well as the day the book was written.
But the primary and most important piece of advice in the book, which is useful online, offline, and even when not playing poker, is this:
When someone is attempting to deceive you about what they're feeling, anything they do consciously is generally going to be the opposite of the truth, and anything they do unconsciously generally reflects the truth.
If you can determine what actions are unconscious (natural) vs concious (artifice), you've gone most of the way to successfully reading the truth.
Support overcards.com by buying it through this link Caro's Book of Poker Tells