I have a shelf of tree books. The guide I grab when I go out to look at trees is Peterson's A Field Guide to Eastern Trees; for me it's the best initial identification guide. This book works well for most of the US; you may want something more specialized if you're in the far west or southeast. If you spend time in cities, I also recommend The Urban Tree Book by Arthur Plotnik.

If you are in the Northeast, I must suggest Bark by Michael Wojtech (www.knowyourtrees.com/). This book is comprehensive and very well laid-out, focusing on identification primarily through the properties of the tree's bark. I like that it gives you the skills to identify trees year-round, which is particularly useful in the winter when many trees are leafless.

If you are in the area covered (Northeast USA), LeafSnap http://leafsnap.com/ is an interesting approach.

There are several good ones. They are reviewed here: http://forestry.about.com/od/aboutforestry/tp/id_tree_guides.htm

Thanks

By far I have found the National Audubon Society Field Guides to be the definative and easiest to use when you are "out there" or have photographed a cool tree for later identification... you can identify trees by Bark, Leaf, and, sometimes Flower. The Field guides have waterproof covers and water resistant pages and last for years... there are different Guides for Eastern and Western regions. Here are the Amazon.com links for each. Eastern: http://goo.gl/QYaNr and the Western: http://goo.gl/snG5r - they (Audubon Society) print wonderful Wildflower books as well. Happy identifying!

Jay Rephan - in Nashville, TN

I purchased the Andriod application Landscaper's Companion for my Android phone. I often use Landscaper's Companion to research a plat or tree I see, mostly because it is with me and available, where often books and guides are not.

I've also used the online internt guides through my phone to help identify items of interest.