Butterfly Field Guides & Checklists: United States
When exploring butterflies in the United States, an excellent overall identification reference is The National Audubon Society Field Guide to North American Butterflies. 
State Specific Field Guides & Butterfly Checklists for the United States
ALABAMA: Field Guide, Checklist
ALASKA: Field Guide, Checklist
ARIZONA: Field Guide, Checklist
ARKANSAS: Field Guide, Checklist
CALIFORNIA: North & South Field Guides, Checklist
COLORADO: Field Guide, Checklist
CONNECTICUT: Field Guide, Checklist
DELAWARE: Field Guide, Checklist
FLORIDA: Field Guide, Checklist
GEORGIA: Field Guide, Checklist
HAWAII: Field Guide, Checklist
IDAHO: Field Guide, Checklist
ILLINOIS: Field Guide, Checklist
INDIANA: Field Guide, Checklist
IOWA: Field Guide, Checklist
KANSAS: Field Guide, Checklist
KENTUCKY: Checklist
LOUISIANA: Field Guide, Checklist
MAINE: Checklist
MARYLAND: Field Guide, Checklist
MASSACHUSETTS: Field Guide, Checklist
MICHIGAN: Field Guide, Checklist
MINNESOTA: Field Guide, Checklist
MISSISSIPPI: Checklist
MISSOURI: Field Guide, Check List
MONTANA: Field Guide, Checklist
NEBRASKA: Checklist
NEVADA: Checklist
NEW HAMPSHIRE: Field Guide, Checklist
NEW JERSEY: Checklist
NEW MEXICO: Checklist
NEW YORK: Field Guide, Checklist
NORTH CAROLINA: Field Guide, Checklist
NORTH DAKOTA: Checklist
OHIO: Field Guide, Checklist
OKLAHOMA: Field Guide, Checklist
OREGON: Field Guide, Checklist
PENNSYLVANIA: Field Guide, Checklist
RHODE ISLAND: Field Guide, Checklist
SOUTH CAROLINA: Field Guide, Checklist
SOUTH DAKOTA: Checklist
TENNESSEE: Checklist
TEXAS: North, Central, South Field Guides, Checklist
UTAH: Field Guide, Checklist
VERMONT: Field Guide, Checklist
VIRGINIA: Field Guide, Checklist
WASHINGTON: Field Guide, Checklist
WEST VIRGINIA: Field Guide, Checklist
WISCONSIN: Checklist
WYOMING: Field Guide, Checklist
