Richard Oedel replies: Two sources, both related. The first is the Forest Products Laboratory in Madison Wisconsin which has been working on wood, wood properties and the like for more than a hundred years. They have many resources that will help you out with questions—www.fpl.fs.usda.gov.
The second is Identifying Wood by Bruce Hoadley. I have that book on my shelf and use it many times a year. It talks about using a microscope to view end grain, but a friend is using his iPhone to get close-up photos of the grain and the results are astonishingly good!