[Amps] W6SAI's Radio handbook

[Hsu]

Hi,
  I just purchase W6SAI's Radio handbook from  Amazon and Yahoo,  one is 20th edition ,the other one is 22nd edition(I think it should be 23rd edition,but not),could someone can tell me what dirffrence between 22nd and 23rd edition? if I own the 2 editions, should I buy  the 23rd edition again? I found 20th published in 1975,22nd edition published in 1981 and  23rd was published in 1997(is that right?).Maybe I should sell one of them and purchase the 23rd edition.
  Thanks!
             73! Hsu