...Mostly Harmless was not published posthumously.
A sixth book in the increasingly inaccurately named Hitchhiker's Guide to the Galaxy trilogy has been announced. Since Douglas Adams ascended to a higher plane of existence almost ten years ago the new book is by Artimis Fowl author Eoin Colfer. The new book is to be called And Another Thing and the only plot details available is that it will contain the same characters.
I've been listening to the radio shows on my commute lately and I enjoyed the early works by Adams quite a bit. However the Hitchhiker's Guide series is a text book example of the declining value of sequels and by the time you hit Mostly Harmless it's ready for bird cage lining. I have no desire to see the new book but given this and the Dune sequels and the Tolkien reworking (which has been going on for decades but there's a recent popular example to drive publishers forward) I am left wondering what dead SF author is next. My money is on Asimov since he's due for a revival and Heinlein has already had his share of posthumous mining.