Not to be missed is The American Heritage Dictionary of Indo-European Roots, Revised and Edited by Calvert Watkins. Originally published separately, it is now an appendix to The American Heritage Dictionary: Fourth College Edition. The appendix cites the Indo-European roots for well over a thousand English words, from the common to the obscure. Both the dictionary and appendix are online at Bartleby. A clear, schematic tree diagram of Indo-European languages is part of the Georgetown Old English Pages. A comprehensive and balanced overview of the field of Indo-European studies is J. P. Mallory, In Search of the Indo-Europeans (London, 1989). Mallory also provides a concise survey of theories on the Indo-European homeland in "A Short History of the Indo-European Problem," Journal of Indo-European Studies 1 (1973): 21-65. The Armenian-homeland theory of Gamkrelidze and Ivanov is summarized in a Scientific American article. W. B. Lockwood, A Panorama of Indo-European Languages (London, 1972) is an excellent general survey of the various languages in the family, and P. Baldi, An Introduction to the Indo-European Languages (Carbondale, Ill., 1983) is a good starting point for delving into Indo-European linguistics. The University of Texas maintains a central resource, including many links, for Indo-European materials. UT San Antonio augments that site with a highly interactive site on Proto-Indo-European. A geocities site has an annoying Flash opening, but some good links.