I am with you in liking the Peterson guide Blast.
I have found a few other good guides out there. The Pojar-Mackinnon Plants of the Pacific Northwest Coast guide is an excellent example. However, like you say, it is area specific and not likely to be much use in the Arizona desert or Louisiana bayous.

And it is so easy for people starting out to get lost in the sea of green that the carefully arranged pictures of the plant in isolation does not help them, the Latin descriptions in the manuals tend to confuse them at first too, especially the kids.

Being shown the plant in the field (woods) at different times of the year seems to be the best way for me.

I find breaking people out of seeing just greenery and seeing the plant is the biggest step.
It is also why I focus on a few of the most available and distinctive plants instead of using a shotgun approach.
I also find the sense of that plant increases once they have tried it. People go from seeing it as just another bit of greenery in the sea of green to seeing it as both a distinct plant and a plant that is quite common. (in fact a common weed, not scarce at all)