How does the book end?

Well there are numerous differences throughout the plot so you'd have to read the book to fully understand it, but basically what happens is Marlee convinces Fitch to wire her the money while utilizing her foreknowledge of a plaintiff verdict to play the stock market with tobacco company stock. She makes more than 10 million on her own in stock trading alone, and Nicolas convinces the jury to return a plaintiff verdict of 9 to 3. Nicolas actually did convince them, as well, and it wasn't kind of a hamhanded "they voted with their hearts" moment. Additionally Celeste Wood ends up being awarded 2 million in life damages plus about 500 million in punitive damages. Nicolas tried to argue it should be a billion, as it would actually hurt the tobacco companies, but that's what they managed to settle on.

In the book it was Marlee's parents who died of lung cancer from smoking. She wanted to hurt the tobacco companies, not specifically Fitch like in the film.

After the verdict, Marlee and Nicolas disappear immediately, and a few months later Marlee shows herself to Fitch one last time, returning his ten million dollars. She tells him she never intended to steal his money, just borrow it, and she only lied because that's all his clients understood, and if there was ever another big trial she and Nick would be watching.

In the book Marlee had very little interaction with Rohr, and never tried to blackmail him in the same way as Fitch. She only made Fitch believe she was talking to Rohr. I think she only speaks to him once in the book, maybe twice, whereas she has constant talks with Fitch. The plan is much more elaborate, and Nick and Marlee are much better at covering their tracks.

So yeah the book is much smarter, but for what it is the movie does well. I think the cast does an excellent job of portraying their characters.