What are the best resources for a beginner breeder?
The best? Go work for an established, reputable breeding farm for a month between January and May. Learn hands-on from experienced professionals in real life situations. Get experience foal watching, foaling, hold for vet exams, handle mares and foals. See what can go wrong and how to treat or prevent it.
Books and online resources can only take you so far. I spent almost four years managing TB mares and foals; I knew next to nothing about breeding when I started, but a great foaling man taught me tons in the first two months.
Are you planning to go into the breeding business or do you want to breed your own mare for fun/ to have a youngster to bring along?
Either way, EventerAJ’s advice is good; but if you can’t go that route, and especially if you plan to be a one time breeder, the book “Blessed Are The Broodmares” is a good place to START.
Finding and building a relationship with a good local repro vet well in advance of any actual breeding is also invaluable.
When everything goes as planned, breeding is fun and rewarding.
But!
When things go bad they tend to go bad very fast.
You need to be able to recognize incipient problems and either know how to help or have a nearby responsive vet on speed dial.
Don’t let yourself be blinded by the hype for stallion X or Y. Do your (www-)research and choose wisely. Breed longvity horses. Talk to old breeders, they have seen everything. And organize a 24/7 vet/clinic you can trust.