Nevada is the best of all Zane Grey novels, but Forlorn River must be read first. I don't see how much the films have in common with the books.

Nevada was my favourite book at one point during my childhood. I found it in the local library, and plodded along just fine without having read Forlorn River first (for reasons beyond my understanding, Nevada was in the children's section of the library and Forlorn River in the adult section, so we only found it later) - I think most of the plot-relevant info can be gleaned without it if I managed it at a very young age. Although the experience would have no doubt been enriched if I had read it first, I think the fact that it does indeed work on its own that may play into it being, as you say, the best of the Zane Grey novels. :-)