It's one of the few instances where I liked the book, but loved the movie. Usually it's the other way round for me.
"Living at risk is jumping off the cliff and building your wings on the way down." - Ray Bradbury
I recently read it and totaly loved it!
Both are among my favorites in each media!
The movie seems to follow the book in many instances line for line and scene for scene. This is probably one of the reasons that author Christopher Bram was pleased with the adaptation and that director Bill Condon won his academy award for "Best Screenplay Adaptation - 1998." There is at least one scene, the "hollywood premiere" scene mentioned in the book that Condon mentions that he couldn't put in the film because of budgetary constraints.
I think the "invisible man at the pool scene" should have been included to compliment the atmospheric, dream-like nuances in the film and to further express the thought processes of Mr. Whale at the time.
The one major change from the book is that the Mexican maid named Maria becomes the Teutonic Hanna which Condon explains was more in keeping with the more probable situation at the time and also because of Whale's connection to Germany through his World War 1 experiences.
I found it to be an interesting touch that Condon designed many of the latter day characters in the film to model the characters in Whale's movies, for example, not only Clay but Hanna and Edmund Kay as well.