I found the versions quite different, but still heading in the same direction. I would get the newest one, but get older ones out of the library to see the difference.
I found the newer edition easier physically to read - it's a well-made softcover, it wears well (from kicking around the bedroom floor or the car, so I have it on hand when I need to refresh my memory!).
There was stuff in earlier editions that didn't seem to have the same focus, in the later one. PLus new stuff I really like, I wish I'd known it earlier.
Mandy, whichever edition you have (and it should say, unles it's the first edition) then try to read the others if you can. But they're all good...
Marg