One of the finest adaptations of a good slim novel I've ever seen was Bill Condon's version of Christopher Bram's book Gods and Monsters (Father of Frankenstein).

Instead of making smart (but major) artistic changes to achieve the same themes/meanings as the source material, as many amazing adaptations have done, Gods and Monsters tried to duplicate the literary material beat for beat, with no bowdlerization, elisions, combining of characters, changes or removal of major scenes, etc. --while also finding a classical visual language which matched the novel's straightforward prose. It's really impressive (--as well as being a movie which, early on, showed that Brendan Fraser was a terrific dramatic actor, playing opposite the world-class Ian McKellen).