k some of it is my mistake and some if it is real differences. (note that I read this 22+ years ago).
I forgot the tower is named Orthanc not Isengard. Isengard is the name of the walled area surrounding the tower.
nevertheless, I thought they flooded Orthanc (like filled up the tower with water), but checking, no they flooded Isengard and were not able to penetrate Orthanc, similar to the movie.
rest of the differences:
movie: the Ents appear to basically walk right into Isengard. they release (apparently) Saruman's own dam and all of the flooding that ensues is basically Saruman's own fault for building his tower in a dammed-up riverbed.
book: the Ents have the ability to carve through solid rock with ease, and tear up the front gate and begin demolishing the outer walls. they demolish almost everything in the Isengard area, but are thwarted by Orthanc's walls which seem to be built of magical rock. after discussing it amongst themselves they spend the night building a canal that diverts the river Isen directly into Isengard, flooding it. ---
