- nice, friendly, pretty, cool, hip
- to be 'in style'.
EDIT Though I was thinking about deleting this answer to avoid more down votes (Ste already provided the correct answer), I decide to leave it as it is for two reasons: to show how not to answer questions in ELU, and to help shed some light on how non-native speakers acquire English vocabulary which might be interesting or useful to some. (But no more down votes, please.)
English learners will run into the word sort soon enough. It is one of the most basic words you need to know (probably in any language). As a beginner, sort = to arrange something in order is enough. Sort of and sort out are usually introduced later once they gets more competent.
Beginners and intermediate learners usually translate everything into their own language before they can fully understand what they read or heard. At some point, with enough proficiency, they will begin to get the meaning of words directly, without having to translate them first. But this is not perfect (and it perhaps will never perfect). To compensate such imperfection, unfamiliar words and idioms are usually substituted by simpler words. For example, sort of might be substituted by kind of, and sort out by solve or resolve, depending on the context.
I myself ran into sorted a few times, and I always substituted it with great. This might not be utterly precise, but it is more than adequate. The first time I learned its meaning, I reasoned with myself that if something was sorted out or someone has sorted out (as in their problems or their lives), it can be said that that something or someone is sorted.
The first time I saw the quote He's got a cushy number. Nice salary, cottage on the estate... he's sorted, I subliminally substitutes sorted with great, as I explained in a now deleted comment:
Someone with nice salary, with a cottage on the estate is surely a great guy to be with. To say a person is sorted in this context suggests that he is a cool guy, a desirable guy.
The best link I got was Urban Dictionary (I couldn't find sorted in any dictionary at that time too). Seeing the definition above was close enough, I hastily posted the link, with my good intention, hoping it can help. Since according to my experience, a question like this has rarely got answered.
The big clue that I really missed was the fact that Sophie Kinsella is an English writer, not an American. If only I knew that I would look up some British dictionary first.