How to find the RRP of books

I've needed to find the RRP of my books for some tax reasons. I was surprised how hard it was but then I came across the Blackwells website. These folks sell books at the RRP. Quite expensive but rather handy if you need to know those RRPs!

I couldn't find all of the books I needed to find on there with the normal search, but the vast majority came up when I entered the ISBN. Very handy.