By Lawrence Newcomb Newcomb's Wildflower Guide: An Ingenious New Key System for Quick, Positive Field Identification of (1st First Edition) [Hardcover]

How to ID a plant. Leaves opposite or alternat? Entire or lobed? Venation parallel or branched? Number of petals? Number of stamens? ETC. Newcomb's Wildflower Guide is, in my experience, the absolute best there is if you can find a copy.