I've only read the book twice and it was a long time ago I last read it but... The whole Saruman/Grima/Theoden thing is way different in the book. In the book by the time Saruman arrives at Saruman's tower, Saruman had escaped (I think by tricking one of the ent, smooth talking it).
Saruman then goes to attack the shire. It's Grima who kills Saruman, but in the Shire.
I don't think Theoden tries to plead with Grima? That's a Peter Jackson / LoTR-the-movie invention no!?
I don't remember enough of the book but the whole "Theoden pleads with Grima / Saruman slaps Grima / Grima stabs Saruman in front of Theoden" is definitely not happening like that in the book.
It's maybe even the biggest difference between the book and the movie (it kinda changes the whole timeline).