The absolute best wildflower guide is Newcomb's Wildflower Guide. Hands down. Really. I'm not kidding. In order to ID a plant, you have to look at three things first:
- the flower type [4 petals, 5 petals, oddly shaped, etc.]
- the plant type [no leaves, basal leaves, vine, etc.]
- the leaf type [entire, toothed, etc.]
One of the most common field guides is Audubon's Wildflower Guide. It's fine, but it's organized by color, then flower shape. Colors are unreliable, especially the pink-purple-blue range. Like all the Audubon field guides, this one has photos of real flowers; however, the photos don't always show leaves and leaves are a very important part of plant ID. We still use this guide, but only as a support to Newcomb's.
We also have the Peterson Field Guide to Medicinal Plants and the Peterson Field Guide to Wildflowers. I don't know if they're good or not because I don't ever need them.
For trees, I use the Audubon Field Guide to Trees and I like it well enough that we've never bothered to get another one. It's divided into leaf types. There's also a section on flowers, seeds, cones, fruit and fall color. Focus on the leaves, go to that section of the photos, find your tree and look it up. I've always found what I've been looking at.
Since we have plenty of mushrooms around here and some of them are edible, we picked up a couple of mushroom guides as well. We like the Audubon Field Guide to Mushrooms very much. They make it very clear which ones are poisonous and just how poisonous they can be. We gather chantrelles with confidence and our kids can use the guide well enough to know what not even to touch.