Hypothetically, say I wanted to make a movie version of the Heir to the Empire series, would Disney be the one who owns the material and I would need to get permission from them or would I go to Timothy Zahns for permission to make the movie?
Disney own the full rights, via their purchase of Lucasfilm
Star Wars: Heir to the Empire is a work of fiction. Names, places, and incidents either are products of the author’s imagination or are used fictitiously. Copyright © 2011 by Lucasfilm Ltd. & ® or ™ where indicated.
All Rights Reserved. Used Under Authorization.
Heir to the Empire - Ballantine Special Edition 2011
Work that is produced under licence rarely requires permission from the author to be adapted into other formats (such as your upcoming film), although their contract may include additional residuals and royalties and the right to be consulted on the use of any characters they have created.