Many useful reference books are now available online:
BuildingGreen Suite provides sustainable "green" design strategies, green product listings, case studies, and access to the publication Environmental Building News. Its also a great resource for anyone interested in LEED Certification of buildings.
The Knovel Digital Library is a collection of over 700 technical manuals, handbooks, and reference books on every aspect of engineering. It includes book chapters, interactive charts and graphs, formulas and equations, physical properties and constants that a scientist or engineer might need. Start your search here: