Here are the areas they test:
http://www.irs.gov/pub/irs-utl/rtrptestspecifications.pdf
While it is "open book" that only refers to Pub 17, the Form 1040 and the 1040 instructions. It doesn't refer to Circular 230. And there were several questions on that. (I took the test 4 months ago & don't remember the specific questions.) Also their versions of the Pub 17, etc. is in a different format than the one you can download from IRS. Same info, just different navigation.
Not terribly exciting, but here's a link to a sample Prometric test - to get a feel for how the computer test works. It doesn't actually help with tax prep questions.
http://www.prometric.com/clientfiles/irs/irsrp/index.htm
I took the NATP test prep class and used their online sample tests for practice. I'm guessing those are only available to people who take the class, but I'm not sure about that. I found taking the sample tests to be very helpful in getting my brain back in a test taking mode.
Good luck.