We are using a free (download of PDF) textbook. You may also purchase the paper version. Each volume is $12-15 (depending on where you get it from). We are going to use almost exclusively volume II. You should also download volume I for reference as well as for one section in chapter 3.
We are using version 3. I do not know anything about differences with version 4, but the page numbers, problems, etc. may be different. Here is the homepage for the textbook where you can find links to all kinds of information.