What is the earliest a litter have been born and survived? One book I read said from day 58, the other said from day 54? 

Lois, my breed often whelps a week early.  I've had quite a few born on day 56, usually from maiden bitches.

I have never had problems when pups have been born on day 56.  However, I had one girl carrying too many pups, and she whelped 10 days early.  I fought and fought to save them, but the lungs just weren't developed enough (it was a heart breaker.)

Most bitches in my breed whelp at around day 59 with no probs at all, some even a few days earlier.

I have a 6 week old litter at the moment.. born 8 days early from 1st mating (day 55).. pups were small, one pup weighed 6 ozs, breed average birth weight is around 16 - 20ozs.. It was very difficult for the 1st 3 weeks. and to be fair, I never expected any of them to survive, let alone all 9 survive and now thriving.. I would not wish a premature litter on anyone..

Earliest whelping otherwise has been day 58, no problems, large litter of 11pups, all decent weights

My experience 56 days and all fine.  Earlier and not really confident of survival.

I had a bitch produce 12 pups (+ 1 born dead) on day 58. She only had 1 mating, tie lasted 12 minutes on day 6 of her season.