Prepare your Google search engine.
Put this string in search box:
‘ -inurl:htm -inurl:html intitle:"index of" +("/ebooks"|"/book") +(chm|pdf|zip) ‘
Hit search button.
The -inurl htm and -inul html is attempting to get rid of regular webpages and show just index pages. Looking for index of in the title is doing the same. Using the pipe ( | ) tells google to look for something OR something else. Here we are telling Google to look for book or ebook directories... and we have listed several common ebook formats (zip, pdf, chf).