I agree with agla and trees that Dirr's Manual (6th edition) is an essential plant identification tool, but I haven't found it to be enough.
If you really want to be able to "ball-park" i.d. any plant that you come across in your horticultural adventures, I've found it's really useful to develop some sense of how a plant identification key works, and what the
Basic terms used in a key mean mean. The Latin can get pretty thick pretty fast, but knowing how to distinguish between simple and compound leaves, opposite, alternate and whorled leaf arrangements, and some of the basic descriptive terms, pubescent and glabrous for example, will allow you to navigate a key successfully and get down to genus if not species.
The way I learned to do this was studying plants native to southern-eastern Wisconsin in their natural setting. My field guide for this was Peterson's "Eastern Trees, Shrubs and Vines". Once I had mastered using that key, I was able to tackle the more diverse herbaceous perennials found round here with another field guide.
Dirr also has a picture guide to Northern landscape species called "Dirr's Hardy Trees and Shrubs" and there is a companion volume for more southern landcape staples. Both are beautiful coffee-table books worth the investment and can really help narrow possibilities in the field as you develop instant recognition.
There is also some great software available. We own Horticopia and use it to i.d. unfamiliar species and cultivars, as well as generate lists and ideas for possible plantings.
Finally, I should confess that I came to landscaping through a passion to know which Oak from which, and that the ability to identify plants off the cuff for potential clients is a great confidence booster for you and them. Nevertheless, I think it is a grossly overrated skill depending on what kind of landscape business you are in (more useful for maintenance than design/build). I would much prefer that I could bid better than I i.d. Oaks.
