Another reason I recommend using Newcomb's Wildflower Guide is that in addition to wildflowers, the guide covers many flowering shrubs and vines. Say you find the following flowering shrub in your travels, and you'd like know its name.
Simply answer the usual questions.
Flower type: 6 Regular parts (6--)
Plant type: Shrub (65-)
Leaf type: Leaves toothed (653)
Since only a handful of plants share this group code, the Locator Key refers us to page 354 with no further questioning. In short order, your unknown plant is found and named: Common Barberry (Berberis vulgaris). As you'll no doubt discover if you practice keying out plants with Newcomb's method, not every identification is this simple as some group codes are shared by many pages of plants that must be narrowed down, one choice at a time.