well its a series of books, so by that I presume you mean the series?
Well mia dumps j.p cause he was using her for her fame. He was the one who ended up calling the paparazzi so they would see them together so he could get his play made into a movie. and when mia is about to leave prom after dumping j.p she bumps into michael and they get back together...
I hope this is descriptive enough?!