An earlier version of the IE Study Center International Trade textbook is available here at this website. It dates to around 2005, but it remains useful because the theory of international trade hasn't changed. Click on the chapter links on the left to access the book.
A more updated and professionally served textbook is available in online, PDF and hardcopy versions at Flat World Knowledge. Flatworld textbooks are affordable, and customizable by adopters. It is a great innovation in textbook publishing that addresses the problem of soaring textbook prices while offering remarkable flexibility for instructors.
If you are an instructor please visit this webpage for more information about the benefits of adopting a Flatworld textbook.