1: A book.
Not only is it a wholly remarkable book, it is also a highly successful one- more popular than the Celestial Home Care Omnibus, better selling than Fifty-Three More Things to Do in Zero Gravity, and more controversial than Oolon Colluphid's trilogy of philospophical blockbusters, Where God Went Wrong, Some More of God's Greatest Mistakes, and Who Is This God Person Anyway?
In many of the more relaxed civilizations on the Outer Eastern Rim of the Galaxy, the Hitchhiker's Guide has already supplanted the great Encyclopedia Galatica as the standard repository of all knowledge and wisdom, for though it has many omissions and contains much that is apocryhal, or at least wildly inaccurate, it scores over the older, more pedestrian work in two important respects.
First, it is slightly cheaper; and second, it has the words Don't Panic inscribed in large friendly letters on its cover.
2: Also a book, written by Douglas Adams, which centers on the book that the book takes its name from. Not to say that the book deals entirely with the book, but that the book is, in the book, a central part. Of the book, that is.