For some reason, that one seems to be a bit harder to find. I have mixed editions; if you want to be consistent in the 1952 edition, that makes it harder. The ones that show up on Chris's link above are the 1971 edition for $39.95 and another for which the edition is not specified for $116.66! Give me a break!
I have two copies, one is the 1968 which is in worse shape than the 1956, so I am not eager to give up the 1956. My 1968 edition is complete except for the first volume, of which I have only the 1948. It doesn't bother me a lot that I don't have consistency throughout the set, but then, I haven't compared the content and don't know what might have changed. Perhaps the later ones had help with the writing. AA didn't exactly write for readability. I read the entire set in 1964, and didn't have a lot of trouble getting through it, but I've heard a lot of bitter complaints from others.