A Goon's Guide To The Galaxy is a 24XX engine game about misadventures in a busy, weird, energetic galazy.
It's 10 pages, and fully self-contained. You don't need to read 24XX to run or play Goon's Guide.
Its layout is also clean and its text is extremely easy to read, but apart from some really cool graphic design elements, there aren't any internal illustrations.
Setting-wise, Goon's Guide does a very good job of capturing the energy of Hitchhiker's Guide in a gameable way, but it's also probably not right to say it's a Hitchhiker's Guide fan game. Goon's works with the same sort of milieu, and you could instantly plug in some proper nouns and make a Hitchiker's Guide campaign out of it, but you could just as easily use Goon's Guide for something like Guardians Of The Galaxy or Star Trek.
Space just has to be big, and cosmopolitan, and balkanized for this game to work.
Mechanics-wise, Goon's Guide is wonderfully simple. You've got all your adventure game basics, with skills at different dice ratings, equipment management if you carry anything super bulky, plus an item-based HP system.
There's also a fairly deep and flavorful character creation system, with skills, classes, homeworlds, unique traits, NPCs, etc. It's simple and quick to operate, but it outputs characters that feel complex and fun to use.
For GMs, there's a little bit of GMing advice, but you're largely expected to seat-of-your-pants it unless you commit to doing your own prep. There are some great, well-written hooks at the back, but no sample adventure or bestiary.
Overall, for galaxy-hopping sci-fi adventure games that are at least a little bit zany, this is the first thing I'd reach for. It's very easy to use, the design choices are all fun and make sense, and it gives groups a lot of freedom within the setting. If anything here sounds like something you like, buy a copy.