Reddit reviews Horror Companion (Savage Worlds, S2P10502)

Just to clarify, there is 1 Core Book (which is, as others have said, all you technically need), and 4 companion books - Super Powers, Science Fiction, Horror, and Fantasy. Then there are numerous setting books such as Deadlands, 50Fathoms (made by Pinnacle), or Beasts & Barbarians (made by the licensee GRAmel).

• Core Book - the stuff you absolutely definitely need
  
• Companion Books - extra optional rules in case you need rules for things like mechas in space (Science Fiction) or sanity rules a la Cthulhu (Horror)
  
• Setting Books - premade worlds for you to play in; often comes with a campaign already made although not always
  

Someone else will say it if I don't, but Savage Worlds has the tagline, "Fast! Furious! Fun!". It's definitely more crunchy than Dungeon World, but manages to make combat go fast.

If you end up getting it, note that the "Savage Worlds Deluxe Explorers Edition" is the newest version.

The books labeled as companions are also up to date. Don't buy anything labeled 'toolkit' as they are earlier parts of what became the companion books.

All of the older versions are extremely similar or literal copies. If you have an old version it'll work with the new stuff as far as I've been able to tell, but that also means its pointless to have more than one 'version'.

Bonus, the system is really quite cheap. You don't need any companions, but they do have a lot of cool stuff:

http://www.amazon.com/Savage-Worlds-Deluxe-Explorers-S2P10016/dp/1937013200

http://www.amazon.com/Fantasy-Companion-S2P10500-Savage-Worlds/dp/0981528112/ref=pd_sim_b_4?ie=UTF8&refRID=1CVET03E63CH10C4H8DD

http://www.amazon.com/Horror-Companion-Savage-Worlds-S2P10502/dp/1937013162/ref=pd_sim_b_2?ie=UTF8&refRID=1CVET03E63CH10C4H8DD

https://www.peginc.com/store/super-powers-companion-second-edition-le/

https://www.peginc.com/store/science-fiction-companion-le-bundle/