Your book preference will depend on how you like information organized and on your level of experience. Here are some titles from my book shelves starting with plant identification references and moving on to gardening with natives references.
One of the best ways to get to know native flora is using plant checklists for specific areas you are interested in. Checklists are usually available at visitor centers or can be found on-line, printed, and brought with you on your excursion.
Sagebrush Country a Wildflower Sanctuary by Ronald J. Taylor. Mountain Press Publishing Company Missoula, Montana 1992. ISBN 0-87842-280-3. One of my favorites, organized by family, lots of good pictures.
Mountain Plants of Northeastern Utah, Andersen and Holmgren. USU Extension, circular 319, Utah State University Logan. Line drawing of each species. My first native plant book.
Alpine Wildflowers of the Rocky Mountains, Duft and Moseley. Mountain Press Publishing Company Missoula, 1989. ISBN 0-87842-238-2 Organized by family, photos organized and cross referenced by color, rock garden plants.
Utah Wildflowers: a Field Guide to Northern and Central Mountains and Valleys Richard J. Shaw. Utah State University Press, Logan Utah. ISBN0-87421-170-0. A good starter book organized by color.
Trees of Utah and the Intermountain West. Michael Kuhns. Utah State University Press Logan, Utah 1998. ISBN 0-87421-244-8.
Flora of the Central Wasatch Front, Utah. Lois A. Arnow, Albee and Wyckoff. University of Utah Printing Service, Salt Lake City, Utah. Technical Key, great for Salt Lake County and the Wasatch Range. No pictures.
Vascular Plants of Northern Utah An Identification Manual Richard J. Shaw Utah state University Press 1989 ISBN 0-87421-141-7. Technical key, good for Salt Lake, summit and Dagget Counties and North. Line drawings of some species.
A Utah Flora. Welsh, Atwood, Goodrich. Higgins. 1993, Brigham Young University Provo, Utah. ISBN 0-8425-2312-8. Technical Key, hard to use but includes all the plants in Utah.
Intermountian Flora. Cronquist, Holmgren, Holmgren, Reveal, Holmgren. The New York Botanical Garden Bronx, New York. Technical Key, the huge 7 volume set with line drawings that answers all the native plant ID questions.
Plant Identification Terminology: An Illustrated Glossary. Harris and Harris, Spring Lake Publishing, Spring Lake, Utah ISBN 0-9640221-5X. Great for understanding technical jargon.
Native Plants for High-Elevation Western Gardens Busco and MorinFulcrum publishing Golden Colorado. ISBN 1-55591-475-6 2003. Great for Gardeners.
WaterWise Native Plants for Intermountain Landscapes. Mee, Barnes, Kjelgren, Sutton, Cerny, Johson. Utah State University Press Logan, Utah ISBN 0-87421-561-7. A must have for native plant gardeners.
Plants for Natural Gardens: Southwestern Native and Adaptive Trees, Shrubs, Wildflowers and Grasses. Judith Phillips. Museum of New Mexico Press Santa Fe New Mexico.ISBN 0-89013-281-X. Great book for gardeners.