Finding ebooks on the internet has never been this easy. Heres a search engine called pdf search engine powered by google which is exclusively dedicated to this purpose. You just have to enter the name of the book and you will find a lot of pdf results matching to your query. You can find any pdf file that you are looking for, that is other than ebooks it can also get you any pdf document available on the net. This feature is also available in google, you just have to type your query plus filetype:pdf to get the same results,  I have referred to it in my earlier post “Some new and unknown features of google“.