Most likely, you will find that what books you find most helpful for plant identification depends primarily on your prior exposure to botanical terminology and secondarily on many other factors, such as whether you intend to lug the book with you into the field, whether you are looking at common or rare species, whether you are happy to just deal with showy flowers or want to figure out what a plant is even when it is not in bloom or potentially even just a bunch of sticks. Because your prior experience and your patience with learning new terms tend to be key indicators of what will be most useful to you, I have organized my favorite books from "easy" to "fiendishly difficult".
All of my favorites listed here share two characteristics that set them apart from the vast mass of other field guides and are, in my experience, a must:
(1) They organize plants with the help of "keys", which means they guide you to the species level through a series of questions that force you to distinguish between one or several possibilities for a particular descriptor of the plant (Are the leaves alternate or opposite? is the stem round or square? Does the plant have thorns or not?)
(2) They present important features of the plant in a line drawing, either with or without color. Drawings foreground the exact features of a plant that make it unique, something that photographs simply cannot do, unless there is an large collection of close-ups and whole-plant shots for every single species -- something that far exceeds the size (and price!) options for most books.
So, if you do not happen to live in the Northeastern US, the specific books below will likely be useless to you, but you may be find equivalent books with these two features for your own geographic region.
Easy Field Guides for Beginners
The "Finder" series, originally published by family-owned Nature Study Guild Publishers, provides an excellent starting point: The slim booklets will slide into your pocket with ease and lead you to identify common species through a set of dichotomous decisions about easily observed plant structures. Because they are not only light but also affordable, you won't mind taking these little guides on a walk with you.
Fern Finder by Anne Hallowell and Barbara Hallowell
Flower Finder by May Teilgaard Watts
Tree Finder by May Teilgaard Watts
Winter Tree Finder by May Theilgaard Watts and Tom Watts
Winter Weed Finder by Dorcas S. Miller and Ellen Amendolara
Intermediate Field Guides for Common Plants
Books in this category are still low on terminology, but they include more species than the "Finder" booklets. Like the booklets, they tend to focus on specific sub-groups of plants, so that they still fit into your backpack or purse (but require you to decide whether you will think about flowers, ferns, or trees on a given hike). I prefer books that offer line or color drawings to those that only give you photographs, because drawings will show you exactly which distinguishing features you need to pay attention to. Before you purchase a guide with flashy photographs and no key, consider that those photographs only show you only a part of the plant (often with no idea what size it even is) and that organizing flowers by color can be mis-leading (where does purple end and blue begin?) and leaves you to randomly leaf through a huge number of species, with no further guidance.
Newcomb's Wildflower Guide by Lawrence Newcomb
For common wildflowers of the Northeastern United States, this guide provides quick and easy identification. The ingenious number key on the first pages quickly narrows down the search based on key features of flowers and leaves, and the language is accessible to non-botanists. All plants are illustrated in color drawings, which allow easy highlighting of key identifiers, a feature that makes this book far superior to guide books that rely on photographs.
A Field Guide to Trees and Shrubs by Petrides and Peterson
Professional Field Guides
The Plants of Pennsylvania by Ann Fowler Rhoads
This is my comprehensive go-to book, because I can count on whatever I come across in Pennsylvania actually being in there. The book is, unfortunately, either intimidating or downright useless for most beginners, because it assumes that you have either an outstanding botanical vocabulary at your command or infinite patience in looking up terms in the book's excellent glossary. It also assumes that you have an excellent hand lens or access to a microscope for assessing the shape of hairs and glands. Furthermore, you should already have some experience with the use of dichotomous keys, or else it's easy to get lost. All that said, this is my authoritative guide to which I refer again and again, and my only savior when it comes to the many species not listed in less comprehensive books geared towards non-specialists.