Reddit mentions of Tree Finder: A Manual for Identification of Trees by their Leaves (Eastern US) (Nature Study Guides)

You could always grab a copy of Tree Finder to carry with you. It's cheap and will actually fit in your pocket.

I am voting with ABITCH and saying Carpinus caroliniana. Carpinus don't usually get so big, but in a city the competition is usually nonexistent and if it can survive the smog, a tree can thrive. The bark seems a little unusual for that species, but I think that's a result of the tree being bigger than an average specimen. It does look a bit like an elm or beech, but take a look at those 'little leaves' developing at the ends of the branches- those are definitely not beech nuts or elm seeds, and they sure look a lot like carpinus inflorescence. They are neat little trees and tend to look very muscular.

Also, I like your blog! I recommend getting this book, and/or others in the series: http://www.amazon.com/Tree-Finder-Manual-Identification-Eastern/dp/0912550015/ref=sr_1_1?ie=UTF8&qid=1310438269&sr=8-1

It will help you learn about the different distinguishing characteristics and walk you through ID's until you get the hang of it. Sometimes picture based guides are not ideal, because one specimen may look very different than it's relatives due to disease or an unideal habitat.

Here is an excellent field tree ID book for the Eastern US by leaves. Here is another tree ID book for the Pacific Coast. Here is a tree ID book for the Eastern US during winter.

It is kind of broken down into many categories and depends on where your interests lay. I recommend DK Press as a publisher of many different topics. I had to use this book for a class last year covering plant propagation techniques and it was very informative.

In terms of general knowledge of plants I would recommend this book. The first section covers the basics of leaf characteristics, identification, hardiness zones, and then the back is all common ornamental plants and tons of information about them.

For quick field identification I would recommend this. Ive had a copy of this book since I was 8 years old and still reference it on occasion. It is very simplistic and quick to use.

Horticulture is a huge subject but those three books, especially the last two, could be very useful in your daily work at a bird sanctuary. Knowing how to identify certain trees to a particular bird habitat can prove to be very helpful.

My knowledge base covers topics ranging from landscape design, turfgrass management, greenhouse design, identification(plants, pests, diseases), propagation, and basic botany. If you have any questions I can try to point you in a good direction to start reading.

EDIT: I just remembered, when I was younger I actually learned tons from the pot growing forums like ICMag and mushroom growing forums like shroomery. While I was never into growing illicit substances, the wealth of information is usually more or less able to be transferred to other plants. Mushrooms/fungi don't especially fall into the realm of horticulture but they are another very interesting subject. The pot forums are great to learn about nutrient schedules, pathology problems, and efficient indoor gardening techniques.