I liked the jail twist better. And maybe I don't think Love is going to be a formidable opponent for him once he decides he's done with her. Her reprieve likely only lasts as long as she's pregnant.
No, in the final two episodes, the series diverted quite a bit from how the book ended probably because it'd be easier to write another season with how they chose to end it compared to how the show tried to end it.
There isn't a third book yet. I know the author was considering writing a third book but it's unlikely that it'll be out before the next season goes into production. I think that's why the show diverted from how the book ended since there wasn't a path, yet, of how to get Joe out of his legal troubles.
But they could come back to the books if she publishes another book before the fourth season. The New York murders, which were touched on more in the book and eventually led to Joe's arrest, could get more focus in the third season but they'll have to create an independent story to fill the rest of the time. The third season could leave off where the second book leaves off.