The novel is well written. It certainly loses some shock and mystery factor as you already know the main events from the movie. It is worth reading, though. The core of the book lies on Father Karras trying to convince himself that he's witnessing an actual demonic possession. The novel quotes several freaky experiments on mind-over-body and non-demonic paranomal events. I wonder if Blatty made it all up or took it from somewhere else.
You have then an atheist mother trying to convince a faithless priest about the existence of the Devil. Quite ironic if you think about it. The novel reeks frantic confusion from Karras side. Gripping.
One more thing omitted from the film: When all the mayhem is finally over, the mother is asked by Karras priest friend if she now believes in God. She denies it, but she admits that she totally believes in the Devil: the Devil keeps making advertisements of his evil and existence. The priest replies: "If the existence of evil demonstrates the existence of the Devil, how do you account for all the good daily deeds?" It's a nice comeback.