Oh man, I am getting old... I remember when this book came out waaaay back in 1991!!! Anyway, amazon.com announced the 20th Anniversary Edition of Timothy Zahn's hardcover novel which, together with Dark Horse's Dark Empire, revived the franchise in the early '90s... The hardcover will be published on June 21, 2011...
Over at the TOS forums, Sue Rostoni confirmed the book:
"Yes, there are plans for a 20th Anniversary Issue of HEIR TO THE EMPIRE, coming out next June. It's a hardcover -- and that's all I can reveal at this time."
Update: starwars.com has an official announcement now as well, adding the following, exciting bit:
The hardcover release will include insightful annotations on the text, as well an exclusive short story by Timothy Zahn featuring his most famous Star Wars creation, Grand Admiral Thrawn.