Square Enix have updated their online bookstore with a page for the first official Kingdom Hearts 3D [Dream Drop Distance] novel. The novel is set to release on June 28, 2012 for a price of ¥800 (approximately US$10.03). It's full name is Kingdom Hearts 3D [Dream Drop Distance] Side Sora. Thanks to aibo_ac7 for the tip.
Update 1: goldpanner has translated the description of the novel provided on the page. You can read this translation below.
Sora, Riku, I would like the two of you to take the Mark of Master Exam.
"Now, Sora and Riku's Mark of Mastery exam shall hereby commence."
Master Xehanort has revived!
Their two journeys to bring peace to the world just barely over, Sora and Riku have received news from Yen Sid regarding a new threat drawing near.
In order to face this new world crisis, Sora and Riku must face the Mark of Mastery Exam.
For this the must go to the 'Realm of Sleep', release the seven Keyholes of Sleep, and gain a new power.
Update 2: Square Enix have created a page for the novel on the Square Enix e-STORE. This confirms the release date and price, and also reveals that the novel has a planned length of 242 pages.
Update 3: Amazon.co.jp have added a pre-order page for the novel, confirming that Kingdom Hearts 3D [Dream Drop Distance] Side Sora will be written by Tomoko Kanemaki, the author of previous Kingdom Hearts novels.