Could be a prequel.
I hadn't realised there were 3 more books after (by a different author mind you).
I can't bring myself to read the other books because all the reviews I've read and heard say they are awful.
I don't think a prequel would work in the Bourne world. His past (the relevant parts anyway) was completely revealed by the end of the third movie.
I'm a huge Bourne movie fan, so I'm excited for another. But I felt the story was pretty much told by the end of the third movie.
I always get nervous when the third or fourth installments come about because they rarely hold up.