I dont recall hearing about this anthology until recently. Did Brandon need a break from writing S3 and so [Arcanum Unbounded] was put in the schedule?
This was put on the schedule when it was determined that Stormlight 3 wouldn't come out until next year. Brandon has been wanting to do something like this for a while, so we went for it.
The book is not just reprinting stuff you've already read. Aside from the new Lift novella (at 40,000 words it's actually a mini-novel), each story has a new full-page illustration and a postscript by Brandon. Each world section of the book has a planetary system map and an essay about the system written by Khriss, who writes the Ars Arcanum section at the end of the books.