Reddit reviews Language

It really depends on your goals, but I assume that you're not looking for the sort of book that is assigned in first year university courses (these aren't necessarily what you want for an overview for the uninstructed).

As Ostrich159 points out, Saussure's Course in General Linguistics is a good choice. To fill this out, an equally good choice that integrates more observation (CGL has a slight theoretical bias), is Edward Sapir's Language.

Note that both books date from the first half of the 20th Century and this is not a coincidence. After the about 1933 or 1957 (the publication Leonard Bloomfield's "Language" and of Noam Chomsky's "Syntactic Structures" respectively), introductions to linguistics also end up being, more and more, introductions to this or that linguistic theory. Starting out with one of the older masters, Saussure or Sapir, ensures that you get something of the feel of the field of study that hasn't been squeezed through the filter of a larger theory about language.

After that introduction, you're then in a better position to evaluate for yourself the more modern style of writing about linguistics (it's not all it's cracked up to be when you only hear voices from within the bubble).