Hi,
Just ran into two apps that I did not know about before. They may be useful to you as artists recreating things from the real world.
1) "Smart app" from iTunes: Tree Encyclopedia .99 cents. A comprehensive interactive app for iPad and phone that "gives visual insights into most ALL existing trees on this planet... with over 500,000 text characters of detailed information."
Given all the talk about making tree models, this may be a good buy.
2) "Desktop app" from the "app store": Essential Anatomy $24.99 (on sale) (also available for ipad). Is a interactive resource intended for medical applications. But would likely be of great benefit to 3D character artists. Human anatomy only.
This anatomy one is of great interest to me personally. Im learning character modeling, rigging, animation, so for humanoid models this is directly useful reference material and information. Basic facts of anatomy provided in the app can also be applied to non-humanoid models. And in the end MUCH cheaper than some text books I was thinking about buying! There are also a number of fee apps for your phone, don't know if they are any good though.