I came across
Boocu in the Feb/Mar issue of a French magazine called
Linux Pratique Essential. After mentioning another e-book search engine
neotake.com (discussed previously in MR - see
https://www.mobileread.com/forums/sho...hlight=neotake), the magazine goes on to say, "On citera également Boocu qui utilise en partie les service de Google Livres et qui a l'avantage de proposer la liste des boutiques vendant l'ouvarage recherché", which I think means something like,
Boocu can also be mentioned, as it uses part of the Google Books service, which has the advantage of offering a list of shops selling the desired book.
If you want free e-books in ePub format, then try
neotake.com.