I'm like you in that I don't know much about it. I do know farmers who will put offspring back to father once, and then no further. I'd been reading recently to try and find out for ourselves as we have now arrived at a stage after a few years where it has become trickier to avoid in-breeding. The idea of it is not something we are comfortable with but it does feel helpful to find out about it from an objective stance. I found this which is only a blog, not a text book but it seems quite neat and at least might start a debate.
"Just as incest in humans leads to genetic defect risks, closely related sires and dams can have defective lambs. One defective gene is normally recessive, but if lambs inherit two copies of a defective gene that runs in the family, they may have serious abnormalities. Inbreeding problems can occur in a small flock that uses its own rams, or gets in rams from another flock which has similar family bloodlines. The maths are complicated, but in simple terms a sheep should not be bred to its parent or offspring, its sibling or half sibling, the sibling or half sibling of its parents or offspring, or the progeny of any of these siblings and half siblings. The closest relationship should be half-grandson to half-granddaughter"