An interesting difference in the book
I just re-read the book and there is an interesting difference I noticed.
In the movie, after Alex gets out of prison and gets beat up by the cops (which the book makes clear is Dim and Billy Boy, not Dim and Georgie as people think) he stumbles to Mr. Alexander's house. At first, Mr. Alexander only knows Alex is the kid who was in the news but doesn't realize Alex was one of the people who attacked him two years earlier. At this point, Mr. A wants to help Alex. Then he realizes who Alex is and turns on him, ultimately trying to get him to kill himself. In the movie, we can only assume that he is trying to get Alex to kill himself for revenge for what Alex did to him and his wife a few years earlier.
But in the book, it is different. Alex stumbles to Mr. Alexander's house the same stuff happens except Mr. A doesn't realize that Alex had attacked him until they had already set up the plan to get him to kill himself. They are doing that (in the book) because they are political activists who want to over thrown the current ruling party. They know that if the Ludivico Treatment turns out to be a failure, it will look bad for the ruling party.
So in the book, it is actually irrelevant that Mr. Alexander realizes that Alex was one of the people who attacked him and his wife. I like that the movie was able to fix that because it seems weird from a storytelling perspective that it didn't really affect anything that Alex had had the previous encounter with Mr. Alexander and his wife.