With a little digging around, I've managed to uncover an excellent fan-translated copy of Aoi Hiiragi's romance comic Mimi o Sumaseba. This is the original "shoujo manga" that served as the basis for Studio Ghibli's 1995 film of the same name.
For fans of Mimi/Whisper of the Heart (and that should be all of you), this is an excellent opportunity to read the original work, and to observe the differences in Miyazaki and Kondo's interpretation. And "interpretation" really is the correct word to apply. While American movies adaptations are expected to follow the original story to the letter (or, at most, refashioned into a Star Wars-style action blockbuster - see Tolkein, C.S. Lewis, Phillip K. Dick), Ghibli's literary adaptations are looser, more conversational between the filmmaker and the original work. Miyazaki, especially, is one to turn a story around to suit his larger agenda and worldview. Howl's Moving Castle is probably the most dramatic example of this.
Mimi the comic is very much in the vein of Japanese romance comics, with all its likeable quirks. Mimi the film is more self-aware, wiser of the ways of love and the tribulations of adolescence. There's a slight undercurrent of sadness that comes from experience. The movie carries a wider arc; it feels more full of life. The comic is purely focused on Shizuku's quest for that elusive, slightly harmless boy.
This fan-translation ("scan-lation") appears on My Whispers, a Mimi/Whisper fan site by someone who uses the moniker Roarkiller. It's obviously a labor of love, and we're obviously grateful for his/her efforts. More recently, Roarkiller has also finished translating Aoi Hiiragi's follow-up manga, which was written after the movie was released. It's a collection of short stories entitled "Happy Times" that follow the exploits of Shizuku and Seiji.
You can download both comics from Roarkiller's Mimi manga page. The page also details some of the differences between the comic and movie versions, for your benefit. I'll put a page from Mimi o Sumaseba below, just so you know what to expect. Happy reading!