A Field Guide to Eastern Trees (Peterson Field Guides) by George A. Petrides, Janet Wehr, and Roger Tory Peterson (1998) A very good guide to trees. Very easy to use.
A Field Guide to Wildflowers: Northeastern and North-Central North America (Peterson Field Guides) by Margaret McKenny and Roger Tory Peterson (1998) Although a book as small as this guide will have to be somewhat incomplete, this is all most of us need. Small enough to carry on a hike and easy to use system for identifying wildflowers. I find this book indispensable for field identification.
A Field Guide to Trees and Shrubs: Northeastern and north-central United States and southeastern and south-central Canada (Peterson Field Guides) by George A. Petrides and Roger Tory Peterson (1973)
Peterson Field Guide to Ferns, Second Edition: Northeastern and Central North America (Peterson Field Guides) by Boughton Cobb, Cheryl Lowe, and Elizabeth Farnsworth (2005)
A Field Guide to Trees and Shrubs: Field Marks of All Trees, Shrubs, and Woody Vines That Grow Wild in the Northeastern and North-Central United Sta (Peterson Field Guides) by George A. Petrides (1973)
A Field Guide to Mushrooms: North America (Peterson Field Guides(R)) by Kent H. McKnight and Vera B. McKnight (1998)
Woody Plants of Maryland by Russell G. Brown and Melvin L. Brown (1972) I think this book is out of print. I like this book although it is probably a bit outdated.
Grey's Manual of Botany by Asa Gray, Eighth (Centennial) Edition expanded by Merritt Lyndon Fernald (1950)- This is a very technical manual, not recommended for the weekend naturalist.
Britton and Brown Illustrated Flora of the Northeastern United States and Adjacent Canada - 3 Volume Set This very technical manual is for the serious botanist. It is a very complete guide to the plants of our area, but like Grey's Manual of Botany it is not recommended for the weekend naturalist. The illustrations are very helpful and make this 3 volume set of books a little easier to use than Grey's Manual.