I have the first (might be second) and the 3rd edition, the third is bigger by a good margin. They have made the pages slightly bigger and slightly thinner in order to keep the book a reasonable size, there are a number of errors but these have been put on a web page. Its slightly less friendly than the other editions, its not as 'matey' and seems slightly more serious. The other editions I always thought of as suitable for clever beginners, the third is more like 1st year uni level.
Its a good book packed with information and a real pleasure to read, although I have the black cover version (not sure version) I am glad I also have the 3rd, personally I think it has more than enough extra info and is done in a different enough way to be worth getting, even if you have another edition.
Somewhere I have posted a link to the website with the corrections on