Even though I never search for online books, you might. I rather have the real thing. Besides, it seems too difficult anyway. So you find a book online… Then what? You read it on your computer? That is too tiring for the eyes. You print it? The price of the ink equals the price of the book most of the time.
Anyway, Google has this site to search for online books. It was introduced last August. They work together with libraries to provide you with books that are out of print or classics in their own right. You can download them in PDF.
If you do not want to Google, you can always try your luck here. The University of Penssylvania has 10,000 books online.