First off, this is the book that inspired the beloved Disney movie Who Framed Roger Rabbit. Despite this, the book and movie could not be any more different. Honestly, I was hoping for some nostalgia, but this book definitely appeals to the adult who likes a good mystery.
There are several differences that were done to make this book more palatable as a movie. First, we all know that in the movie, we see cartoons in the real world that cannot die and are impervious to injury. In the book, the cartoons in question are comic strip characters that even speak with thought bubbles and can die the same ways a person can, so the toons can make doppelgangers of themselves to do the stunt while the original is safe.
Other differences that were made by Disney to make a more family friendly story were more obvious. Valiant remains an alcoholic, the mob is involved, and pornographic strips are even mentioned as a major element of the book. I can see why these changes were made, but it was surprising to open the book and see the distinct lack of whimsy that Disney had imbued this story with in the first place.
Finally, Disney kind of changed the genre of this from a noir mystery set in the 1920s to a more modern comedy romp. Personally, I preferred the noir feel of the novel to the comedy of the movie, but again would have liked some of the more comedic elements to have been there.