Newcomb's Wildflower Guide. Lawrence Newcomb. 1977. Little, Brown, and Company. 490 pp. (paperback)
A Field Guide to Wildflowers of Northeastern and North Central North America (Peterson Field Guide). Roger Tory Peterson and Margaret McKenny. 1996. Houghton Mifflin Company. 420 pp. (paperback)
A Field Guide to Trees and Shrubs (Peterson Field Guides). George A. Petrides. Second Edition. 1986. Houghton Mifflin Company. 428 pp. (paperback)
The Shrub Identification Book. George WD Symonds. 1963. William Morrow and Company. (paperback)
Weeds of the Northeast. Richard H. Uva, Joseph C. Neal, and Joseph M. DiTomaso. 1997. Comstock Publishing Associates, a Division of Cornell University Press. 396 pp. (paperback)
Preliminary Checklist of the Vascular Flora of Connecticut. Joseph J. Dowhan. 1979. State Geological and Natural History Survey of Connecticut. Report of Investigations No. 8. Connecticut Department of Environmental Protection. 176. pp. (paperback; may be purchased from DEP at 79 Elm St., Hartford, Connecticut 06106  424-3555)
Manual of Vascular Plants of Northeastern United States and Adjacent Canada, Second Edition. Henry A. Gleason and Arthur Cronquist. 1993. The New York Botanical Garden, Bronx, NY. 910 pp. (hardbound; the most up-to-date complete flora)
A Field Guide to the Birds (Peterson Field Guide). Roger Tory Peterson. Houghton Mifflin Company.
The Atlas of Breeding Birds of Connecticut. Louis R. Bevier, editor. 1994. State Geological and Natural History Survey of Connecticut Bulletin 113. Connecticut Department of Environmental Protection. 461 pp. (hardbound; has distribution maps)
Butterflies Through Binoculars, The East. Jeffrey Glassberg. 1999. Oxford University Press. (paperback)
Dragonflies Through Binoculars. Sidney W. Dunkle. 2000. Oxford University Press (paperback)
Reading the Forested Landscape: A Naturalist's History of New England. Tom Wessels. 1997. The Countryman Press, Woodstock, VT. 199 pp. (paperback)
Both the National Audubon Society and the Peterson field guide series include additional books on birds, butterflies, wildflowers, reptiles and amphibians, rocks and minerals, trees, insects (and spiders), mushrooms, etc.