As others have said, I also recommend Types and Programming Languages.

> What's the relationship between programming language theory and type theory?

I'd say programming language theory is more broad than type theory, for instance topics such as compilation techniques and runtime systems are more distant from type theory (though you might prove some type-theoretic thing like type preservation).

There's also a range of type theory books because it's a field that spans pure logic to programming languages, so you can find books like Type Theory and Functional Programming by Thompson that elaborate things like dependent types early on.