You can't wander into the speculative future (or alternative past) blindly – you need a guide to show you the way. These five game manuals are the best at explaining aliens, mutants, angry robots and even non-sparkly vampires.
Call of Cthulhu RPG: Malleus Monstrorum. With the subtitle, "Lore of Things Beyond," you know this book won't steer you wrong. This is actually a revised edition of the Call of Cthulhu RPG's Creature Companion. It collects all the statistics and background information on the many Lovecraftian horrors that have appeared in every published Call of Cthulhu book ever. From crazed cultists to tentacled things to elder gods your mortal mind can't even comprehend, Malleus Monstrorum has you covered.
Star Trek Roleplaying Game: Aliens. There are a lot of aliens out there, and some of them aren't even part of the Federation. In your explorations, you will likely encounter many of them. Therefore, it behooves you to know as much as possible about their abilities, habits and physiology. This sourcebook for the Star Trek RPG fills in all the details. Curious about Horta culture? Wondering how much to tip a Betazoid waiter? Most of these questions are answered here. This also makes a great companion piece to the Starfleet Operations Manual.
World of Darkness: Armory Reloaded. This is, essentially, a book of violence. Not only does it provide a list of efficient and brutal weapons to use in the battle against vampires, werewolves, and other supernatural creatures (or on their behalf, if you swing that way), but it also amplifies the World of Darkness combat rules, making them…well, more violent. When I reviewed this book for Robot Viking a few months ago, I called out this paragraph from the intro, which perfectly exemplifies the approach taken throughout the book:
We want to emphasize here and throughout: combat is some scary business. Blood spattering in the mud, people screaming, the smell of cordite burning nostrils. Bombs blowing people to bits. A vampire's claws leaving a man with his guts hanging out and his wife standing ten feet away, crying so hard she breaks a rib.
AD&D 2nd Edition: Van Richten's Guide to Vampires. They aren't here for you to fall in love with, they don't generally smell nice, and they sure as hell don't sparkle. In the many worlds of Dungeons & Dragons, vampires are universally evil. They're usually placed at the apex of the undead hierarchy (liches sometimes compete for the top spot), so you'll find them behind nefarious schemes as often as they're responsible for a rash of bloodless corpses. Van Richten has hunted and slain vampires of all kinds, so you'd do well to heed his advice. Don't bring garlic, bring an enchanted sword. Or, better yet, a Daylight spell.
Gamma World: Machines and Mutants. Gamma World has gone through so many editions and revisions it's hard to keep track. This is from one of the more recent incarnations of the post-nuclear apocalyptic setting. Imagine a world that's a little bit Logan's Run, a little bit Fallout, and that's pretty much Gamma World. This is perhaps the most bizarre manual on this list, as it documents all manner of weird robots and freakish mutants. No, I mean really freakish. Sentient, evil penguins. Genetically engineered fireproof bears. Exploding fish. My personal favorite is the Architect, a robot that's been continually building for decades, creating non-sensical road networks dotted with houses that will remain forever empty. Also, there are man-eating cars.
Don't say I didn't warn you.