Non-electronic references--
Barnes, Burton and Warren Wagner.  Michigan Trees. 1992. The University of Michigan Press. Ann Arbor, MI.

Fernald, M.L., 1950. Grays Manual of Botany. American Book Co., New York, 1632 pp.

Gleason, Henry A., 1974. Illustrating Flora of the Northeastern United States and Adjacent Canada. 3 Volumes. for N.Y. Botanical Garden, Hafner Press, New York.

Muenscher, W.C., 191950. Keys to Woody Plants. Sixth Edition. Comstock Publishing Assocs., Cornell University Press, Ithaca, NY, 107 pp.

Petrides, G.A., 1986. A Field Guide to Trees and Shrubs. Peterson Field Guide Series, Houghton Mifflin Co., Boston, MA, 428 pp.

Symonds, G.W.D. and S.V. Chelminski, 1958. The Tree Identification Book. Quill Press, New York, 272 pp.

Core, E.L. and N.P. Ammons, 1958. Woody Plants in Winter. The Boxwood Press, Pittsburg, PA.

Vitale, Alice Thomas.  1997.  Leaves in Myth, Magic and Medicine. Stewart, Tabori, and Chang. New York.

Cool web sites that were a source of knowledge and inspiration--
http://bluehen.ags.udel.edu/ugarden.html
http://www.wisc.edu/botany/
http://www.isc.tamu.edu/FLORA/
(http://www.csdl.tamu.edu/FLORA/res/)
http://www.knoware.nl/flora/
(European Flora: Click here to check it out!)