It is a testament to the quality of these books that even now, decades after they were published, they remain the benchmark against which all SF humor must be judged. Don't start these books expecting rigorous hard SF; you won't find it. What you will find is a romp through space-time that is nothing short of hilarious.
This, the fifth Hitchhiker's book, is something of a let-down. It's not bad, per se, and there are some really funny moments in it. But on the whole, it's a much darker and more serious book than the rest of the Hitchhiker novels. It seems like Adams wrote it just to shut up the people clamoring for another Hitchhiker book; an impression that's definitely furthered by the ending (which, not to give too much away, makes a sequel damn near impossible). Read it, but don't expect it to measure up to the first four books.
A lot of Douglas Adams fans were disappointed by this book, largely because it wasn't a Hitchhiker book. I wasn't one of them. If this book isn't as laugh-out-loud funny as The Hitchhiker's Guide (although a lot of it is, particularly the Electric Monk), it was certainly more tightly plotted. Not one event in this book is anything less than crucial to the plot. Pay attention while reading this book, or you're not going to understand it. And once you finish it, avoid the much-inferior sequel like the plague.
These books are classics of SF, and deservedly so. The first three books (which are actually collections of short stories, a la The Martian Chronicles) are some of the best early SF out there. Read them. Edge was written much later, but (IMHO) retains much of the feel of the earlier works. There are other sequels, and while they're still decent books, they're not up to the high standards of these earlier works.
Like Asimov's Foundation books, the first robot books are SF classics. They deal with a detective, Elijah Bailey, and his android partner, R. Daneel Olivaw. Asimov manages to deftly combine genres with his SF mysteries. Many people consider these to be as good or better than his Foundation books, but they never quite grabbed me in the same way, though I certainly enjoyed them.
These light fantasy books chronicle the "myth-adventures" of Skeeve, an inept young magician, and Aahz, his demon mentor. The first four are slapstick, humorous adventure, similar in tone to Brust's early Vlad Taltos books. As the series progress, though, it gets more serious and less funny. And, somewhat annoyingly, the plots are no longer resolved in a single book. If you like humorous fantasy, give these a shot.
Similar in tone to the Myth books, these deal with the ultra-rich commander of a band of misfits in the Space Patrol, or some such organization. Despite -- or because of -- the cliched premise, the books are actually humorous. Not as outrageously funny as Adams' Hitchhiker books, these are still very much worth reading.