The Road to Dune by Frank Herbert, Brian Herbert, & Kevin J. Anderson

The Road to Dune covers so much of the Dune universe that it's hard to say where to start, there is the alternate Dune book Spice Planet where we see a very different but familiar characters talking and dealing with new characters, there are also correspondents between Frank Herbert and several authors around the time of the publication of the original novel and the sequel Dune Messiah. There are also deleted scenes from those books too plus there are new short stories that take place during the Legends of Dune trilogy. I highly recommend this book and suggest that you should pick up a copy.