In American English it means , I know how do you feel or think.

I agree with DanN. (I will call Speaker 1 "Edward" and Speaker 2 "Bella" to make it easier; I think this is from "Twilight"). Bella is annoyed because she is unable to hide certain feelings/reactions, and Bella's mom thinks that Bella is easy to read. When Edward says, "I find you very difficult to read" he means that he is looking deeper, that she is more complicated; he also means that he cares about her, and he is not satisfied with only knowing her superficially; he wants to truly know her. Bella is grateful to Edward for paying attention, for looking deeper; here is someone who cares about who she really is and not just about her embarrassing reaction to whatever just happened. It is also comforting to her to know that she is not being judged just by that recent incident; he values her. I also agree with Sonia; Bella's comment, "You must be a good reader" is not what anyone expects her to say.

He asks if he's annoying her, which means there was something on her face that told him that she was annoyed. She then got frustrated that she let that show on her face, so she says that her mom calls her an open book. He says that she's actually quite hard to read, it was just this one moment that he found he could read her, so she says "you must be a good reader then" because if she's hard to read, he just read her.

It likely means that the person considers themself to be more than might meet the eye, but one would not notice this unless they are more attentive. In English the popular expression is "don't judge a book by its cover"

Person 2 probably says that since they feel that Person 1 can read them, (understand how they feel), even though Person 1 thinks Person 2 is hard to read.