Reddit mentions of Critical Theory Today: A User-Friendly Guide

If you want really excellent intro books then I definitely recommend Lois Tyson's Critical Theory Today. This has all the really important schools that are important right now, except eco-criticism, which is kind of a bummer. But I think the latter edition hits a little bit on it under postcolonial theory. This is a good intro text that has overview of what's going on.

Norton Anthology of Critical Theory was mentioned, and while this is an excellent anthology, it's huge and can be a bit complicated to read the actual source material without knowing about it before hand, but it's pretty nice being able to read the actual texts of different theorists. Similar to this is Julie Rivkin and Michael Ryan's Literary Theory: An Anthology. While Norton's goes chronologically all the way back to Plato, Rivkin's text groups all the texts under the major schools so you get a comprehensive view of each one. It's worth mentioning though that Norton does have a secondary Table of Contents where they group the readings under school as well.

You mentioned wanting to know postmodernism, and that's another thing that Tyson's text doesn't include, as it's more of a movement instead of a criticism. An intro text similar to Tyson's that does include eco-criticism and postmodernism though is Peter Barry's Beginning Theory.

If you have very little knowledge of theory and criticism, I'd really recommend picking up Tyson's book and reading that so you get an overview of the text before moving on to an anthology. Like I said, the texts can be incredibly dense and difficult to read, and if you've never been exposed to them before it'll just make it even more difficult. Tyson's text also has suggested readings under each school as well to expand what you're reading.

I used Barry as well. I also suggest Critical Theory Today by Lois Tyson.