Well, you could just try googling them. However, using a simple query for the book's title might not get you the expected results in most cases, therefore people came up with more specialized queries using Google's advanced query operators. Surely, you could also search it in your favorite bittorrent sites if you don't care about possibly "extra-legal" findings.
The solution. I'd like to recommend you a tool that makes finding ebooks easier: eBook Searchr! It's an e-book search engine based on Google's Custom Search technology and it currently searches more than 200 selected and categorized e-book sites, including the ones mentioned above. Although the default settings contain "extra-legal" results as well, if you don't like to see those, you can restrict your search to the "Public Domain" area or to "Commercial" stores only. Anything you find on eBook Searchr, you can also find it googling. However, the difference with this custom search engine is this:
1. The search is kept within relevant sites,
2. The results are filtered out of spam,
3. and you can find that specific e-book in less time!
That specific e-book is, of course, my free e-book!