Weeds of the Northeast by Richard H. Uva, Joseph C. Neal, and Joseph M. DiTomaso is one of those reference books that never goes out of date. My copy, which I only purchased a few months ago, is already well-thumbed.
I have yet to find a book about weeds that is as comprehensive and as inclusive as this one. And while it focuses on weeds that are primarily found in the northeast quadrant of the country (north to Canada, south to Virginia, and west to Wisconsin), my guess is that it would be helpful for many other areas of North America as well.
The book is designed in such a way that identification of weeds can be accomplished relatively easily. There are identification tables that differentiate similar weeds based on characteristics and multiple pictures showing each of the plants included at multiple stages of their life cycle.
I find myself walking through the gardens with the book in hand, quickly identifying things that have eluded my efforts to properly name them for years. Of course, one woman's wildflower is another woman's weed, and so my lone cautionary note is that not everything in the book needs to be pulled up and relegated to the compost pile.
That said, the information about each plant's growth habits, propagation, and root system has made it easier to eradicate weeds that are not welcome in our perennial beds and has helped us to effectively control those that we've chosen to cultivate and enjoy as "wildflowers".