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.

This has totally been my experience with bird guides, and now I'm kicking myself for not thinking of that with 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

Especially if you add some kind of location/region ("Texas"; "New England"; "Pacific Northwest"; "Baltimore").