Shokhet, a user of this website, mentions in his bio that when he cites שמירת שבת כהלכתה (hereinafter שש”ך) that he uses the second edition. What (other than the cover) are the major differences between editions of שש”ך?
Apparently, the first edition allowed using the water from the Dood Shemesh on Shabbat.
This turned out to be very controversial, so in the introduction to the 2nd edition R' Neuwirth זצ”ל just mentions that he didn't bother republishing everything.
Seems he wanted the Sefer to be usable by everybody and saw no reason to prove his point or pick a fight.
I have never seen the first edition - seems impossible to find.
(The 2nd volume came out years later and mainly discusses Kiddush and Havdala.)
(The recent "fat" edition was updated with modern technology issues and facts of life - and references the classic second edition chapter-n-verse in the margin so that students could use both versions in class.)