Endurance 101 is a new book that has just been released, looks to be a good one.
Oh and any horse CANNOT do endurance. I get tired of hearing that. While individuals from many breeds can finish, lots and lots of horses simply cant and never will be able to.
A horse needs to be completely sound but mental aspects are a big factor.
Most athletic horse in the world isnt worth a fart in the wind if it wont drink out of a creek, quite a few horses simply wont eat and drink when stressed. Ive seen Arabians with pulse rates of 48 spike to 70 as soon as they come into he vet check area out of nerves, again great athlete, pulses down fine alone, but can never get through a vet check. I could go on there are plenty of things that could keep a horse from doing endurance.