I have no photo, but I had a mayfly light on me on the Beaver River in Pennsylvania. It was an adult with brilliant neon green eyes, with the same color on the body. I've looked through photos on the website, but didn't find anything with those beautiful eyes. I'm not a fisherman, just a bug lover, and am curious if anyone has an idea what it was. I have ID books on larvae stages, but I guess I need to buy a book on adult stage
Well, you certainly didn't provide sufficient descriptive information on which one could base even a reasonably reliable guess, but that rarely stops me from attempting to make one.
So, I would say you might have been visited by a Maccaffertium mediopunctatum mediopunctatum male dun, previously classified as Stenonema mediopunctatum, and sometimes referred to as an Olive Cahill Quill.
There are many good books, and which one is the best choice for you depends on many factors. On the pragmatic end of the scale are some small hatch guides which feature the more likely aquatic insect emergences you are likely to encounter streamside. Other books exclusively feature one insect order, like mayflies, for example. Other books cover all aquatic insects, but usually not to the species level.
This is a subject that is dear to my heart, and I wrote an article about it titled Entomology Bookshelf in the December 2006 issue of Hatches Magazine. My suggestion is that you read the article before deciding which book or books to purchase.