The first edition of PLAI has a nice introductory chapter on types that explains this notation and has a whole section on type inference. It's also freely available online, so that's nice. (chapter 10 is on types) I haven't looked at the second edition at all yet, but the chapter on types looks like it has more code than math.

Benjamin Pierce's Types and Programming Languages seems to be the gold standard of intro type theory, and definitely covers this notation and the math behind it, in a much more rigorous fashion than PLAI. It is a much deeper book though, so it doesn't get to type inference and Hindley Milner until later. I've read some of it and haven't actually gotten this far yet, but can say that it is a superb textbook, it's very well written and easy for an amateur to follow, as long as you're relatively comfortable reading and writing proofs. (but you don't need more math than your standard undergrad discrete math course)

You might also want to look at Lambda the Ultimate's Getting Started page, which has a wide range of suggestions covering more than just types, but programming languages in general.

http://lambda-the-ultimate.org/node/492

http://www.cis.upenn.edu/~bcpierce/tapl/

http://cs.brown.edu/~sk/Publications/Books/ProgLangs/2007-04...

Another excellent and up-to-date resource is Practical Foundations for Programming Languages by Robert Harper.

http://amzn.com/1107029570

Harper hosts an online version of this book: http://www.cs.cmu.edu/~rwh/plbook/book.pdf.

I learned from Foundations of Programming Languages by John C. Mitchell. However, it is a very dense book and I wouldn't want to work through it without a partner or a class.

http://www.amazon.com/Foundations-Programming-Languages-Comp...

Hindley's Basic Simple Type Theory is accessible and comes from the horse's mouth: http://books.google.co.uk/books/about/Basic_Simple_Type_Theo....