Hi, Daniel. I might be able to offer a better recommendation if I knew a few more things, such as your primary disciplinary interest, previous level of exposure, etc.
In lieu of that, I'll make a general recommendation: Postmodernism by Kevin Hart
Postmodernism: A Beginner's Guide (Beginners Guide (Oneworld)): Kevin Hart: 9781851683383: Amazon.com: Books
One thing I like about Hart's book is that it recognizes that postmodernism has played out differently both geographically and by academic field. I think Hart gives a good topography. He also addresses religion toward the end of the book, which I think is better than ignoring completely or having it dominate the conversation.
If you want to read a postmodernist, I think Michel Foucault is one of the more readable and useful:
The Order of Things is probably the work that most established him as a philosopher. If you're into politics and science, one highly interesting work on postmodernism by a postmodernist is
The Postmodern Condition by Lyotard. If literary theory is your thing, You can try reading Derrida, but you'll have to like playing games with language.