Why separate books?

As I had just said in my previous blog, I have finished reading book 1. However, while I was reading further, I am confused to why there are separate books. The story seems to take off exactly where it had left off in the previous book and it seems very pointless to have this. But at the same time, the author would not seperate this story into different books for no reason. There must be something behind it I believe. As I look into it, the location does not change so locations is most likely not a reason that there is a separate book. The characters stay the same and the central character remains Jake so a shift in characters is not a reason for the new book. However, one thing that I did notice is that Brett left Jake at the very end of book 1. On page 63, Brett says to Jake, "Oh don't be a fool. I am going away to-morrow." And her leave is confirmed in book 2 because the very first sentence of book 2 is "I did not see Brett again until she came back from San Sebastion." So maybe this might have something to do with it but I'm not sure.