Modern Marvels: World Trade Center (DVD) -- This is the original documentary that aired on the History Channel in 2001 before the attacks. It was initially called "Modern Marvels", but has since been renamed "In Memoriam". It's a fascinating, comprehensive look at the center's construction and inner workings. Because it was made before the attacks, it's one of the few documentaries that spends time focusing on the history rather than the tragedy.
The World Trade Center: Rise And Fall Of An American Icon (DVD) -- Another fantastic History Channel documentary on the towers.
The World Trade Center Remembered (Paperback) -- A beautiful book filled with spectacular images of the pre-9/11 WTC. Photographs by Sonja Bullaty and Angelo Lomeo. Text by Paul Goldberger, architecture critic for the New Yorker.