I think the most important aspect of "A Clockwork Orange" is it's use of language as a literary tool. It's also key to its misunderstanding, as it tends to lead towards secondary interpretations (i.e. Kubrick's film) more than the text itself.

In addition to freewill being a key theme in the book, the nadsat language is used to disassociate the reader from the actions and events partaken in by the narrator. Alex describes in clear detail the fact that he's beating the crap out of old ladies in the book, but you, the reader, don't interpret it that way. Those acts are hidden behind a language that doesn't hold the same connotations to you, the reader, so you don'tt look at Alex the same way because of it.

Conversely when his punishment and "re-education" begins you view that as harsh and inhumane, because those things are expressed in more familiar terms simply because the reader is more comfortable with the language Alex uses. Worse, he had something that he loved taken away from him (his classical music), which you understand wholeheartedly as the reader.

The fact that Alex punishment was arguably justified (if not grotesque in it's own way), is something that's missed because language disassociates the crime and amplifies the impact of the punishment. It makes you question the humaneness of criminal punishment, because it's expressed from a perspective rarely portrayed.

All of these things are completely lost once the story is taken into any visual medium. The idea of making it a film was flawed before Kubrick even touched the project, and any other director would have struggled to have the same impact as the book. I genuinely think that the project attracted Kubrick for the wrong reasons and he was far more interested in making something provocative, which "A Clockwork Orange" had plenty of opportunities.

If you haven't read "A Clockwork Orange", I strongly recommend you do, and I strongly recommend reading it as quickly and in as short a timeline as possible. Understanding the language is key, and it makes it a slow read at the start. The quicker you can become familiar with it, the easier the rest of it goes.

Then after you're done reading it, reread it again. Being familiar with the language from the start makes some of the more graphic scenes at the start really stand out in a way that they don't the first read through.