While not being exactly about type theory like Pierce books (you can't waste your money on those), I really like the part about types in "The Implementation of Functional Programming Languages" by Peyton Jones (the chapters about type system are written by Peter Hancock). It's seen mostly from a technical point of view. The book notably contains a fully explained implementation of a type checker.

You can check it online while waiting for the Pierson ones: http://research.microsoft.com/en-us/um/people/simonpj/papers...