I want to study type system theory. I don't have any background in type system theory so I'm more or less a beginner (except the articles I've read on the subject and which I find intimidating because of the notation used). What are some good books that could get me started? I was looking at Benjamin C. Pierce's "Types and Programming Languages" book. Is that appropriate for a beginner or is it too difficult? If it's too difficult what else could I use to start my study?
I think Types and Programming Languages would be a good choice.
When I've first read TaPL, I was a beginner in type theory as well, and I didn't find it too hard to understand.
You will see a lot of the notation used in the articles you've seen, but the notation will be explained before it's used, so it shouldn't be a problem. There isn't really anything intimidating about the notation once you know what it means1.
1 Which is quite often the case with these things. Sometimes I think that most of mathematics and formal computer science consists of inventing scary-looking notations for simple things, so the uneducated masses don't realize how easy it all really is.
TAPL is a hard book to understand and is aimed at graduates (and possibly phd candidates). It's certainly not for beginners. I would instead recommend a book like 'Concepts in Programming Languages' by John C. Mitchell aimed at undergraduates to get yourself familiar with the basics first.