I have seen an edition of The Hitchhikers Guide To The Galaxy that was a compilation of four books, The Hitchhiker's Guide To The Galaxy, The Restaurant at the End of the Universe, Life, the Universe and Everything, and So Long, and Thanks for All the Fish.
The book described itself as A trilogy in four parts, but after I finished Life, the Universe and Everything and started So Long, and Thanks for All the Fish, it's quite clear that the original series is only a trilogy, and the fourth book is just an epilogue, of sorts.
When I searched for the series on the Kindle Store, there were compilations claiming to have five or six books in them. Ever since then, the question has bugged me: how many books are there, officially, in The Hitchhiker's Guide to The Galaxy series?