How to search for free books on internet?

Here is a simple trick for finding free ebooks on Google with extensions .chm,.pdf,.zip.Its not targeted for any particular book.Copy the text of link below to your Google search box.

inurl:htm -inurl:html intitle:"index of" +("/ebook"|"/book") +(chm|pdf|zip)

The above query removes all the html pages and searches for folders like ebook or book and search for file types chm, pdf and zip.You can add more extensions to the list by editing the above text,

Note: You may find copyrighted content or illegal books for which Im not responsible