Osprey Books are usually first in line - but can be more expense, although you find them in reasonable libraries. I think there was a title on armies of the HYW.
You might also find something like a Funcken book, not sure if they did one for HYW although there is a generic ancient to Napoleonic one.
As another option to Osprey, there were a series of magazines (del Prado?) about medieval warriors, used to come with a pre-painted figure. Quite a few magazines covered HYW subjects - Agincourt, Crecy etc, and they crop up on ebay . Not as good as full Osprey titles, but ok for a low price. Hanover Antiques in Scarborough usually has a box full for 50p a copy. (Also Napoleonic & WWII ones so it takes quite a while to rake through the box).