Jstor, the ISI data-bases, google scholar and the like all are incredibly helpful to actually search for papers and even provide means to save you the tedious work of typing over all the bibliographic details. Depending on the way in which you organise your research, zotero may become an invaluable helper, too - especially as it retrieves details for books from their page at amazon. For, obviously, books hardly ever feature in databases focusing on journal papers.
Sometimes, however, you just want a bibtex entry for a book and then taking a detour via zotero can be nearly as annoying as taking the scenic route (i.e. just typing stuff over). For these cases, I just came across lead.to/amazon/en/ (via this post - as usually while searching for something else totally unrelated to this issue: http://tex.stackexchange.com/questions/143/what-are-good-sites-to-find-c...). It lets you search amazon and shows a pretty complete bibtex entry generated from the amazon metadata. For example, searching for "haraway witness" on amazon.co.uk via lead.to gives you:
@BOOK{Haraway199702,
title={Modest Witness: Feminism and Technoscience},
author={Donna J. Haraway},
publisher={Routledge},
year={1997},
month={2},
edition={1st ed},
isbn={9780415912457},
url={http://amazon.co.uk/o/ASIN/0415912458/},
price={£24.99},
totalpages={376},
timestamp={2011.02.13},
}
Searching amazon.com gives you the US edition. I admit that the title is not 100% correct, but then again I chose a title very difficult to properly translate to bibtex on purpose, so I guess I shouldn't complain about missing "@"s and "_"s too much.
This is going to save me precious minutes in the months to come - I just wish I'd known earlier...