There are quite a few non fiction writings that were in OMWF that will probably be in the new book as well, mainly forewords to other books such as Brewers Phrase and Fable Millennum edition to give one example although there are several more. There is also the article he wrote about the development of the Discworld which has only been published in the proof copy of Sourcery which may well be included (or at least Colin hinted it would be in an email last year). We may also see transcripts of speeches included. I'm looking forward to it