The main ebook format is .pdf, but ebooks also are delivered as .chm, doc and even .txt format. That means we will have to include at least pdf, doc and chm to get good results in our searches. Ebooks are unfortunately sometimes packed, that means you probably would like to include zip and rar as well to your search

(pdf|chm|doc|txt|zip|rar)

To get good results we need to exclude the following -inurl:htm -inurl:html

To search for directories that contain ebooks you use the following search string.

-inurl:htm -inurl:asp -inurl:html (“index of|"last modified"|"parent of") AND ("ebook"|"ebooks"|"book"|"books") AND (pdf|chm|doc|txt|zip|rar) AND "Oct-2005"

You should really add another keyword, a title, author or publisher to get better results than that general search. Simply add the keyword at the end of your search using +”keyword”

Update: Here are explanations for the commands listed on this page:

• -inurl:htm -inurl:asp -inurl:html (This one forces the search engine to only include pages in the results that do not have htm, asp or html in their urls.
• (“index of|”last modified”|”parent of”) AND (“ebook”|”ebooks”|”book”|”books”) AND (pdf|chm|doc|txt|zip|rar) AND “Oct-2005″ (a large command with several operators tied together. It basically tells Google to only include pages in the results that contain at least one phrase of each, for instance index of AND ebook AND pdf AND Oct-2005.

You can naturally modify the parameters to suite your needs better. You could for instance change the date, remove a phrase or add a new file extension to the mix for more versatility.