Sadly, I don't have a great source for the kind of not-very-showy weedy flowers - you notice in my notes here I mention there are some of them that I was using names like yours for, for ages.
I have a large collection of wildflower guides, mostly from used book sales, and I tend to go through them all when I need a new ID. However, I will say that the Audubon guide (which is the only one I had for a long time) is *particularly* bad with this sort of thing (partly because the photos it uses don't work very well for plants with tiny, nondescript flowers, partly because a lot of them just aren't in it.)
The Little Golden Guides - the ones that are smaller than a paperback - have a "Weeds" volume, and it's a good place to start - it doesn't have everything, and the only real way to use it is to thumb through it, but it's small enough that you can, and it it has a lot of the more common weedy flowers. It's also written up in such a way that reading it will give you a good start on knowing where to look in other books. I've also heard good things about Wild Urban Plants of the Northeast: A Field Guide, which is a much larger weeds book that only came out very recently and I haven't gotten my sticky little hands on yet.
In general though in wildflower guides, I'd look for one that uses color drawings instead of/alongside photographs - I've found drawings are a lot better at emphasizing the right things with those kinds of plants.
The other thing: you would be surpised how often it works, but if you just go to google image search, and stick in a simple description - like "common purple flower weed" or something - you'll often come up with a photo of your plant, and you can then go to the website and find an ID. (Also, if you've got a tentative ID out of a book, google image searching the scientific name will get you a lot more photos you can use to confirm/deny.)
The other thing you might want to do is google/wiki the scientific names listed in this post. :P I'm not really writing a guide here (yet) but I betcha that over half of the stuff you're seeing is also on my list, and you'll probably know it if you see it. I was going to link to ID pages so people could do that more easily, but I ran out of time last night - maybe I'll do it later.
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I have a large collection of wildflower guides, mostly from used book sales, and I tend to go through them all when I need a new ID. However, I will say that the Audubon guide (which is the only one I had for a long time) is *particularly* bad with this sort of thing (partly because the photos it uses don't work very well for plants with tiny, nondescript flowers, partly because a lot of them just aren't in it.)
The Little Golden Guides - the ones that are smaller than a paperback - have a "Weeds" volume, and it's a good place to start - it doesn't have everything, and the only real way to use it is to thumb through it, but it's small enough that you can, and it it has a lot of the more common weedy flowers. It's also written up in such a way that reading it will give you a good start on knowing where to look in other books. I've also heard good things about Wild Urban Plants of the Northeast: A Field Guide, which is a much larger weeds book that only came out very recently and I haven't gotten my sticky little hands on yet.
In general though in wildflower guides, I'd look for one that uses color drawings instead of/alongside photographs - I've found drawings are a lot better at emphasizing the right things with those kinds of plants.
The other thing: you would be surpised how often it works, but if you just go to google image search, and stick in a simple description - like "common purple flower weed" or something - you'll often come up with a photo of your plant, and you can then go to the website and find an ID. (Also, if you've got a tentative ID out of a book, google image searching the scientific name will get you a lot more photos you can use to confirm/deny.)
The other thing you might want to do is google/wiki the scientific names listed in this post. :P I'm not really writing a guide here (yet) but I betcha that over half of the stuff you're seeing is also on my list, and you'll probably know it if you see it. I was going to link to ID pages so people could do that more easily, but I ran out of time last night - maybe I'll do it later.