A novelised version of 5cm was first serialised in the Da Vinci magazine between May and October 2007 (i.e. issues no. 157 - 162) which included a number of sketches in them. A hardback release was then made available on December 14th. It is considered a "light novel" in Japan due to the fact that it's short and is related to an Anime piece of work.
On May 20th 2011, a second 384 page paperback novel subtitled "One More Side" was released which is written by Shinta Kanou, the author who wrote the novels for Voices of a Distant Star and The Place Promised in Our Early Days, has written a different version of the novel.
As you might have already guessed from the subtitle, it's a different take on the story in which Kanou describes as "It's just like how the story develops one way in the novel version and then another in the Manga version of Voices of a Distant Star. It's kind of like parallel worlds." Instead of viewing the story from Takaki's point of view, it's mostly from Akari's this time.
Unfortunately as far as I'm aware, there aren't many official translations at the moment (at least not English) so, you will just have to bear with my fan translation at the moment. A Simplified Chinese fan translation is also available for those of you interested and an official Thai translation was released with the "Makoto Shinkai's Memory Box".
Just before the second novel was published, there was also a coloured Manga drawn by Yukiko Seike and the serialisation started in July's issue of "Afternoon" in 2010. As of now, you can purchase standalone copies of the Manga although don't expect the artwork to be as breathtaking at the Anime!
As you may have read from the introduction, Makoto Shinkai had brainstormed many ideas by writing a number of short stories before he picked three of them to produce 5cm. One of those ten draft stories that Shinkai wrote but never made it into the final production of 5cm was "The Sky Outside The Window" which anyone who either bought the limited edition DVD from Japan or the Taiwanese collector's edition had a chance to read.
Maybe you'll notice some similarities between Miyuki and the characters in the movie.
If you wish to get a copy of the Japanese novel, copies can be found on Yes-Asia or HMV Japan.