In the past, I've mentioned Doabbooks.org and OAPEN.org as good search engines for Open Access books.
This afternoon, I looked up something on WorldCat (WorldCat.org) ("the world's largest card catalog"). I noticed, for the first time, when doing an advanced search for a book on WorldCat, that a person can limit search results to Open Access titles.
In the recent past, there was some inconclusiveness about this, but it seems to me that now "Open Access" universally implies "free". Being a lover of all things of value that are free, this piqued my interest.
Yes, card catalogs only give the holdings of one or more libraries, not of books that have been published, per se. However, WorldCat claims to show the holdings of "over 10,000 libraries worldwide".
Doabbooks.org and OAPEN.org are not exhaustive search engines. It occurs to me that a person might be able to eke out one or more Open Access titles, than he or she otherwise would, by searching WorldCat. Then he or she could use a general search engine like Google or Bing to locate where that/those title(s) can be downloaded.