Recently I bought the novel version of Howl's Moving Castle. The movie version is very different from the novel, yet the two also share a lot of similarites. Both version follow a very similar occurance of events but they both tend to differ on these events (in the movie when Howl has a temper tantrum he isn't really clothed where was in the book he is. Both versions involve green goo). If you love the movie and are afraid that you'll constantly be thinking "that did not happen in the movie" or vice versa PLEASE give this book/ movie a chance any ways because both are really good. The same author wrote two sequels- Castle in the Air (which Ghibli's Castle in the Sky is not based on) and House of Many Ways (released in 2008). House of Many Ways is known as more of a sequel than Castle in the Air.
One of my favorite Ghibli films (watched it again last night) Whisper of the Heart is based on a manga called Mimi wo Sumaseba that was released for only a few short months. Whisper of the Heart's sequel, The Cat Returns, has a manga adaptation (meaning the movie came out first) called Baron: The Cat Returns which is illustrated by the same manga-ka as Mimi wo Sumaseba. I have yet to read either of the manga versions, but I probably will soon since the movies are amoung my favorite Ghibli films.
Grave of the Fireflies is based on novel of the same name.
Kiki's Delivery Service is based on the first novel in a series of the same name. The series was recently revealed to have a conclusion released October 2009 [source].
My Neighbors the Yamadas is based on a yonkoma manga series called Nono-chan.
Nausicaa of the Valley of the Wind is based on a manga of the same name.
Only Yesterday is based on a manga of the same name.
Porco Rosso is a lose adaptation of Miyazaki's manga Hikotei Jidai.
Tales from Earthsea is losely based on books in the Earthsea series.