As you know Google.com is the most popular search engine in the world.

Here are some tips to helps you find eBooks with Google:

Find Apache's (default) Index page

Try this query:

+("index of") +("/ebooks"|"/book") +(chm|pdf|zip|rar) +apache

Find a particular eBook file

Try this query:

allinurl: +(rar|chm|zip|pdf|tgz) TheTitle