Cambell, J. Duncan.  Aviation Badges and Insignia of the United States 1913-1946.  1977.  The Triangle Press.

Carroll, Warren.  Eagles Recalled.  Air Force Wings of Canada, Great Britain and the British Commonwealth 1913-1945.  1997.  Schiffer Publishing Ltd.  Atglen, PA.

Chalif, Don and Bender, Roger. Military Pilot & Aircrew Badges of the World (1870-Present). Vol 1, Europe (Albania-Hungary).  1982.  R. J. Bender Publishing.

Fitzsimmons, Charles.  Thirty years of U.S. Army Pilot Wings.  1917-1947.  1994.

Huff, Russ.  Wings of World War 2.  1985.

Maguire, Jon.  Silver Wings, Pinks and Greens.  1996.  Schiffer Publishing Ltd.  Atglen, PA.

Maguire, Jon.  More Silver Wings, Pinks and Greens.  1997.  Schiffer Publishing Ltd.  Atglen, PA.

Morris, Terry. United States Army Air Service Wing Badges - Uniforms and Insignia 1913-1918. Scott A. Duff Publications

Rosignoli, Guido.  Air Force Badges and Insignia of World War 2.  1976. Arco Publishing Co. New York.

Other tools:

Loupes - A 10X loup (small magnifying glass) is one of the best tools for viewing details of a badge.  Can be found just about anywhere and are worth 100X their weight in gold if you are collecting.

Calipers - Useful to have around to check with known original wings.  Because a cast wing is almost always slightly smaller than the original die struck piece, it can help you determine a good from reproduction wing.

Scales - In general, original wings will be a different weight than reproductions.