Franchise Starts hilarious, turns into something wholly different
Far be it from me to knock the holy grail of satire, but the first two books, which were adapted completely from the original radio series, are the absolute best. The radio show was more about cohesive episodes than cohesive seasons, which makes for a fun read. After that, the books were written as books, and they start to get decidedly less madcap and more plot-driven. Of course, they're ridiculous plots, but the actual jokes get farther and farther apart, Guide asides become rare, and once you realize it, you may feel duped.
The first two books are based on the first season of the radio series, with Restaurant at the End of the Universe appropriating a handful of gags from the second season. Such as there's a plot, it merely serves the jokes, which are hilarious.
Life The Universe And Everything is okay. Just okay. It has some elements lifted from the second season of the radio series, but it has a definite plot. Knowledge of the game of Cricket is highly recommended. This book was adapted for the radio in the 2000s, and the adaptation suffers because many of the good bits from the book were used in the earlier radio series.
So Long And Thanks For All The Fish was created entirely out of whole cloth, and is entirely focused on Arthur finding True Happiness. It's much less grandiose than the previous, but it's even less compelling than the first two. It's a feel-good read, though. The radio adaptation leads directly into the next story, which is a slight improvement.
If you haven't read Mostly Harmless yet, do so only with extreme caution. It's a horribly bleak book that's basically the polar opposite of the previous. The radio adaptation finishes much better.
If you don't watch the television adaptation for any other reason, watch it for the terrible effects. Zaphod's other head came off a mannequin and is normally "asleep."
The Movie brings old and new concepts together fairly well, but may not be very accessible for non-fans. Your Mileage May Vary, but I thought the opening sequence was hilarious, and the love subplot well-done and a well-needed fix (not that I don't like Fenchurch...)
Franchise The first five books
This review excludes And Another Thing by Eoin Colfer, the sixth canonical book.
What's there to say about The Hitchhikers Guide To The Galaxy? Well, it's probably the funniest thing this troper has ever read. The humor's "angle" seems to stem primariy from taking ordinary Earth niusances and situations and blowing them up to galactic proportions as a way of making a point about the fallacies of the human condition, though there's also a liberal helping of wordplay and all-around silliness. The first two books are light on plot; they seem to set up a wide, broad plot, most of which is scrapped by the time they're finished, but the author manages to be funny enough, cryptic enough, and profound enough to make it all fall together.
The third book is the only one that seems to have a definite beginning, middle, and end. It follows up somewhat broadly on the events of the first pair, but the style rings true and the characters are the same people you remember. This one expands upon characters who had relatively minor roles and paints a more expansive picture of the universe they all inhabit. For edge-of-your-seat action, Life, The Universe, and Everything is the way to go, but at the end of the day it's all a big vehicle for zany jokes.
The fourth book is something of a black sheep, so much so that throughout most of it it feels like an entirely different series altogether. It's a love story at heart, and deals with one of our protagonists, Arthur, coping with the events of his first three adventures as he tries to win the heart of his soulmate. To compensate for the change of pace, though, the fifth is nonstop, wacky science-fiction... albeit much, much darker than what we've seen before. The upside is that Adams is back to perfect form in both storytelling and humor, even if his sentences get even more convoluted than they had already been. The ending is a bit of a downer, but those who don't mind that Eoin Colfer took over the series after the unfortunate death of Adams, there's always And Another Thing. Which isn't half bad.