This book is a compilation of several magazine articles, interviews, short stories, speeches, and other general writings by the late Douglas Adams, released to commemorate his death.
It also contains the beginning of the book which would (perhaps) become The Salmon of Doubt — the third Dirk Gently novel. A fax from Douglas to his agent suggests where he was heading with the plot, but the story is frustratingly unfinished (it cuts out, halfway through) and we will never know what happens next.
Of course, now that they have published this book, it is safe to believe rumours to the effect that they have published this book. Just don’t expect a sixth Hitchhiker's novel.
Early editions of the book included [[The_Private_Life_of_Genghis_Khan?]], a short story by Douglas which appeared in The Utterly Utterly Merry Comic Relief Christmas Book. However, as this was based on a sketch co-written by him and Graham Chapman, copyright issues reared their head and later copies do not include it. This is no great loss, as the story may be read for free (with pictures) here.