The original manga of The Cat Returns by Aoi Hiiragi has a much more subtle, evocative message than its movie adaptation. The manga's primary theme involves coming to terms with the loss of a loved one, and it has a more magical realist touch that blurs the line between inner and outer. However, the movie solely places emphasis on believing in oneself, which feels trite by itself. It, thereby, changed significant plot elements, which degraded Aoi Hiiragi's original numinous vision. Moreover, the manga has many oneiric scenes that the movie left out, some of which tread on metaphysical territory that is difficult to discuss without spoiling. The manga has a high-art vibe that makes you question whether our inner worlds could be considered "alive" and what it means to bond with other beings; it effectively conveys such profound, deep questions through its use of ambiguity during key moments, giving it an ethereal and mysterious atmosphere. The manga somewhat resembles a story by Kenji Miyazawa.
Overall, I recommend the manga. The movie did a disservice to Aoi Hiiragi's vision. It is very hard to appreciate a movie adaptation when you see how superior the original source material is. Granted, I still recommend watching the movie but only after you've read the manga. It is nice to compare and contrast them. Finally, on a humorous note, the title "The Cat Returns" doesn't make sense in the context of the movie's plot, but it does makes sense in relation to the original manga.
The manga is one volume and pretty short. My criticism of the movie is that it changed the fundamental meaning of the story. The movie did adapt pretty much everything from the manga, but it changed some significant plot elements.