I did not find other postings about this so I just wanted to mention the trail guides from the Grand Canyon Association. I only got one, the North Kaibab guide for $4 including shipping, but I really like it. It is pocket-sized and only 31 pages but is pretty interesting. The info that you really need to do a hike (distances, elevation etc.) can easily be found online, but this goes into detail about flora, fauna, natural and human history, and especially geology along the whole trail, breaking it down into sections. I have read several different guides and a lot online, but had not found a lot of this info. If you just want to know the essentials about a hike, there is no need to get it, but if you want more detailed background or are a "science-minded" person it is well worth the $4, part of which goes to their conservation projects. I'm sure the guides for the other trails are just as good and they also have a lot of other products.
I am looking forward to having this in my pocket as I hike so I can know what I am looking at, although another guide I have includes more info about specific species. This one does not picture every species and only goes into detail about a few, but it does list what plants and animals are in each section and has several pictures with captions so it shouldn't be too difficult to identify most just with this guide. The detail about the geology and the natural history stand out to me, though, one fact in particular: that there are marine fossils along the route even very near the rim, from a sea that was there hundreds of millions of years ago. Hard to imagine.