I just managed to loot some precious planes books,but the previous owner while carefull did leave some of them in humidy place, and some pages are welded, any tips how to carefully separate them without damaging those books ?

I just looted Wings of Fame and World Air Power journal,yay ! Thick heavily ilustrated aviation books !

I would try to carefully pry them apart with a long and thin knife.
(Like one these long utility knives where you break off segments)
Keep the pages parallel and close to eachother while you gently push the knife between them.
That way the knife won't dig into the paper.

If they're welded shut, have you tried an acetylene torch?

Try to pry them apart over boiling water (steam). Be careful though, may ruin it.

He must have been a real fan of aviation, thats all i'm gonna say
but I agree either with Piper, either steam them apart, or try wetting the stuck paper again and carefully prying them apart, and keeping them apert until dry.

SD