I'm working on a story that involves two species of the same genus that can interbreed, but it is rare and difficult. The Panthera hybrids are similar to what I'm thinking about, particularly the way in which imprinted genes lead to ligers being huge and tiglons being of a more modest size. Let's also say this genus is mammalian and Haldane's rule applies so that male hybrids are sterile.
So, my questions:
Does anyone have good books to recommend on hybridization like this?
Assuming a recent overlap in territory, what factors could contribute to these two species being hypothetically interfertile (at least as far as their female offspring go) and yet finding it prohibitively difficult to successfully interbreed? For the purposes of this story matings occur, but live birth is rare.