It's in beta version at http://scholar.google.com/ but it's a Google search engine that searches "specifically for scholarly literature, including peer-reviewed papers, theses, books, preprints, abstracts and technical reports from all broad areas of research." For example, I did a search on "andragody" and it returned 2,000+ hits. No big surprise really. The main search engine of Google returned even more: 90,000+. But what's different about Google Scholar is that it also returns back some hits as "books" and ranks the return results differently. For example, here's the first hit on the Google Scholar result list:
[BOOK] The modern practice of adult education: from pedagogy to andragogy
MS Knowles - 1980 - Wilton, Conn.: Chicago: Association Press; Follett Pub.
Cited by 261 - Web Search - Library Search
Notice it returns Malcolm Knowles' important work from 1980 and states that this "book" has been cited by 261 other sources found in this 2,000+ hit list. You can actually click the "Cited by 261" text to receive the list of 261 source articles. Very interesting. It actually ranks the search results using this citation number. Documents that are cited more frequently appear at the top of the list.
Now The modern practice of adult education: from pedagogy to andragogy is not yet available electronically (Note: this may change soon as Google makes plans to digitize library voumes through there Google Print service! WOW!) but Google Scholar at least points you to its existance as a seminal document in the field. So at that point you switch gears and do a little leg work at your local library to find the volume.
This search engine could be very useful especially to junior university and college students who are new to a specific field.