As an alternative to paper books out on the trail, MyNature has developed an iPhone app that looks great. Check out the demo videos on YouTube.
The tree guide features over 200 trees commonly found across North America.
The animal tracks guide featues the ablility to identify from the smallest Weasel to the largest Grizzly Bear found throughout North America at the touch of a button.
Since I don’t own an iPhone or iTouch, I can’t try it out myself. Let me know if you have tried one of these apps.