Book Theoden was simply consistent, never as weak or conflicted. Much more dramatic in the movies. Heck, in the book
he's ready to ride out and meet Isengard exactly like they counsel him to do in the movie, and it's actually Gandalf who plays the role of telling him to cool his jets and go to Helm's Deep instead. Kind of an interesting 180 for the sake of making him appear a little weaker as a leader in comparison to Aragorn, but it also gave him a cool arc of redemption as well.