Thinking about starting it. Seems like it might be a pretty advance read for a casual reader like myself. Got interested in it after watching The Ninth Gate and would like to just hear more about the story.
It's hard to say. I've read it and I'm not a casual reader and I did have to reread certain parts to understand things. It's not necessarily a bad thing though. One huge tip I could offer is just to skip the parts dealing with the "Dumas" plot line (which is the main plot of the book lol). It's boring and doesn't tie in at all with the (much better) subplot that everyone is interested in--the evil book inspired by the Prince of Darkness himself.
Actually, if you can rent the ebook from your library (most do now), just do a search for "the girl" and read all those sections. The rest of the story basically follows what you already saw in the movie. It's only the conversations between Corso and the girl that are different (and yes, they do add a lot of interesting backstory). And the finale of course. That is different as well, sort of...the same basic situation happens, but it's still different.