With that in mind, I wanted to have a tree identification reference with me as well. So I looked, both for iPhone Apps and on the Kindle store for paper reference books that had been converted for Kindle use. I'm sad to say I didn't find anything that would work for me.
There were a couple of iPhone Apps, but they were too limited for what I had in mind. Maybe the North Woods tree identifier would work every now and then, but not down here in Texas.
On the Kindle store, I browsed on the web and clicked the 'send a sample' link for a couple of them. Unfortunately, I mostly got academic texts with long introductions. If there were useful tree identification pages in those books, I couldn't tell it from the sample section, and they were too expensive for me to risk buying sight unseen.
I think there's a real market here that seems unfilled. Tree books, flower books, field and meadow guides of all kinds–but designed as ebooks with a good index up front to help someone standing in the mud to make an identification quickly and move on. I'd even buy a well-designed iPhone App to serve that purpose, but please make it as easy to use as iBird.
Now, if I've just overlooked the perfect solution for my problem, be sure to drop me a comment and point me in the right direction.