I happened to come across a copy of the novelisation of Revenge of the Sith
for 2 bucks. Given it's a favorite Star Wars movie of mine, I bought it and read on the bus to work.
And so it was that I learned a few things.

Dooku had big plans to reform things for the greater good but didn't realize he was only the pretend apprentice to Darth Sidious til the very end.
Oops.
C3PO's memory was ordered to be wiped as he suggested in front of Bail Organa he would tell Padme's children about their parents one day. This was not in the film as far as I recall - so it kind of explains why the action was taken - however it doesn't explain why R2D2 didn't really say much about his experiences to Luke!
I had thought the duel between Sidous and Yoda ended in a bit of a draw with Sidious taking the moral victory. The novel reveals during the fight Yoda realised he had lost the fight before he had even been born.
Which is kind of amusing as Mace Windu actually beat Sidious and had him till Anakin stepped in. Unless Sidious was foxing but the book didn't suggest that.
Which is kind of amusing as Mace Windu actually beat Sidious and had him till Anakin stepped in. Unless Sidious was foxing but the book didn't suggest that.
The novel confirmed Obi-Wan was indeed a bad ass. But we all knew that anyway.
In the A New Hope film, Obi-Wan discusses Vader's being seduced by the dark side. In ROTS, the seduction is played as an obvious slow build. My take on the novel is Anakin is more tricked by a promise of saving Padme's life. One can argue the toss but the book really gets into Anakin's emotions earlier on than the namesake film, meaning his seduction is more believable.
Of course one must take these things with a grain of salt as Star Wars canon is the films themselves (and anything else official since the release of The Force Awakens), anything else is expanded universe stuff which at the end of the day is 'nice to know' things and adventure but it doesn't cut it as stuff that's an immutable law of the franchise.
Generally speaking the novel was pretty faithful to the film, as you would expect. It gave some great insights into how the Jedi's perceived the Force as they went into battle and the like.