Searchlores has compiled on one page the search engines for several ebook archives (Project Gutenberg, UPenn, etc.). The page comments are full of additional links, tips, and hacks. At Searchlores' targets you can also find advice on finding music, images, laws, passwords, barcodes, and several other search objectives.
For texts in other languages, see Intratext. The extensive archive focuses on a broad number of languages, including minority languages.
OpenSourceShakespeare has a nicely designed Shakespeare archive complete with full text and concordance search capability. OSS also provides the option to see all the lines for a single character.
If you're looking for more eBooks, see this previous post.