The series Dark Angel (starring Jessica Alba) was cancelled mid-storyline. Is there any official information about what would have happened if the show continued? (For example, novelisations, statements from producers/writers).
Yes. There were 3 novels written: a prequel titled Before the Dawn and two books that pick up where Season 2 left off, Skin Game and After the Dark.
There's been talks on and off about some sort of continuation, possibly in the form of a movie, but given how long it's been...it seems unlikely anything will happen.
A few online groups have tried to raise support for a continuation. Barcode Day is one such effort. Otherwise, the only official ending is what's written in the books. (...or fanfiction, if you refuse the book endings.)