3 Differences Mark the Splits between Books 1, 2, and 3

I see three major differences that mark the splits betwen Book 1, Book 2, and Book 3.

1.  The change in books mirrors changes in the phases of Jake's relationship cycle with Brett.
      In Book 1, Jake feels very attached to Brett but relatively hopeless that their relationship will work out.  This is evidenced by his time alone in his room: "Then I couldn't keep away from it, and I started to think about Brett. ...Then all of a sudden I started to cry" (39).  Yet at the same time, Brett and Jake appear to be relatively close both mentally and physically (they kiss a fair amount in these scenes).
      The end of Book 1 sets up for the beginning of Book 2.  Jake tries to kiss Brett a second time, whereupon she pushes him away with, "Oh don't!" (71) and leaves saying "I won't see you again" (71).
      The fact that Brett leaves Jake physically at the end of Book 1 marks the distance in the relationship between Jake and Brett that seems to be present in Book 2.  On the first page of Book 2, Jake says, "Brett was gone" (75).  Jake and Brett have no intimate interactions throughout Book 2.  Brett neither strokes Jake to settle him down or vice versa.  Nor does Jake really say much to Brett directly during the bar scene in which Mike is introduced, or throughout the entire fiesta, especially considering the long duration when the two were in the same places at the same times.  Brett is dealing with other men (Cohn and Mike and Romero) ahead of Jake and does not seek much social support from Jake during this time.  Their one-on-one conversation in this scene is basically Brett soliciting Jake's help in going off with Romero.  The fact that the two are spending alone time together does not mirror Brett's admiration and love for Jake in and of himself.  She is using him here to help her.
      The end of Book 2 is the epitome of Jake's feelings of apartness from Brett.  Brett has explicitly left him for another man, a man that Jake probably looks up to.  Not only is he a bullfighter, but he is attractive, has the ability to have children, and is kind, understanding, and passionate.  Moreover, Brett likes him.  When Brett pursues Romero, Jake purposefully gets "drunker than [he] ever remembered having been" (227).  When Brett leaves with him, he felt like "about six people were missing" (228).  He feels awful about the distance between them.
      Book 3 marks the beginning of the upward slope in the relationship between Brett and Jake.  During his alone time, Jake has a break from Brett, during which he is not explicitly happy.  Yet he does revel in the fact that France is "the simplest country to live in" because "if you want people to like you you only have to spend a little money" (237).  He might be ironic here: no amount of money could buy him a happy life with Brett, which is the relationship that he would really like to buy with money.  And yet, it is his steadfast quality of loyalty to Brett that brings him back together.  After all of Brett's disloyalty to everybody, she calls Jake back as the only one who she knows will permanently be loyal to her.  Thus, their relationship reaches a point where the two become mentally and physically more intimate again.  Again, they both think about a life together.

2.  A change in book marks the entrance and  fact that a significant change in setting will be taking place.  Book 1 largely takes place in France, whereas Book 2 takes place in Spain.  The end of Book 2 marks the end of the Spanish Fiesta scene; at the beginning of Book 3, Jake leaves Spain (though he returns to Spain upon Brett's call, the difference in Book is still necessary because of the major change in scene that is set up at the beginning of Chapter XIX).

3.  The change in book also marks the change in the groups of people with whom Jake will interact.  In the first book, Jake must interact with a wide variety of people from all over Paris simply because he spends most of his time travelling all over the city to drink, socialize, and do some work on the side.  Book 2 singles out a certain group of characters who is especially volatile when placed together.  Since they are isolated in Spain together, Jake is only communicating with select bullfighters and a group of men who are all fawning over Brett (he never actually says anything to Bill's girl, to my recollection).  In Book 3, after quickly saying goodbye to some other people, Jake has his first daylight hours of alone time, followed by his first daylight hours of one-on-one Brett time.