First pup in my first home bred litter was tail first, and about twice the expected size for a Bedlington pup. He seemed to take ages, but was fine. Once I knew he was proper breech, and that the sack was broken, I got the KY jelly in there, which helped a lot, and also managed to bring the back legs into the proper position. He was born very shortly after with a little help from me. On reading about more births on this board, and comparing with subsequent litters, the length of time he took to be born wasn't long at all for a first pup.
The rest of the pups were also huge, but came either back legs first, or head first. Apparently huge pups is normal for my lines :-)
The Book that helped me most besides The Book of the Bitch, was
The Rearing and Whelping of Puppies mine is spiral bound, very useful for keeping by the whelping box. Very graphic, but clear pictures and language.
The likelyhood is, that veterinary intervention would be too late in the case of a proper breech pup, as by the time you know it's breech, it's nearly there! The most important thing is to recognise a problem immediately, and know what to do. I had already been involved in small animal breeding as a teen, and then Pedigree cat Breeding as an adult, before having my first litter of pups ;-) I'd also spent 3months living in a NICU and learning heaps about caring for early newborns. Not an experience I'd wish to repeat, but educationally invaluable and the skills are all transferrable :-)
Far too many people go into breeding with little or no knowledge, of what is likely to occur, so well done you for trying to gain some insight :-)