I read this book for my senior seminar, which was thematically "medieval romance," and we examined the ways in which ATPH exhibited some of its key characteristics.
Yes, tough prose--little punctuation--but certainly not as difficult as, say, Faulkner, though it's been compared to that quite often.
The story itself is unimpressive to me; it resounds on a different realm than plot. I think it successfully captures the feel of the decay of the old west particularly well, though.