title_body: Many sites propose free or cheap ebooks? Which are legitimate? Looking on the web for some ebooks for sale in the right format (EPUB
  for me), I have been amazed by the number of sites pretending to offer
  them for free download, even though these are in copyright commercial
  books. Some others offers books at a very low price.
  
  I am aware that some authors (notoriously Cory Doctorow, for example)
  allow that, but it seems still to be the exception rather than the
  rule.
  
  My last attempt was when searching for "The A.I. Chronicles" Peralta epub
  
  What is hiding behind these sites?
  
  How do I know which sites are actually conducting a legitimate business?
  
  For example, I found on http://www.mobileread.com a pointer to a site
  https://www.humblebundle.com/, selling book bundles at a low
  price, for example a sci-fi bundle. Though old, they are all books under
  copyright, AFAIK. 
  
  How do I know whether the site is conducting a legitimate business? What are signs that distinguish a scam from a legitimate low price seller?
upvoted_answer: The short answer is that any website offering permanent access to large numbers of copyrighted books, for very little money, from multiple publishers is unlikely to be legitimate.
  
  HumbleBundle offers its bundles for limited time.
  
  Book Barbarian, ManyBooks, BookBub, Luzme, and no doubt many others, search the web on your behalf and find books that are currently being offered from reputable sites, like Amazon and Kobo, for little or nothing.  Almost all of their free books are available only for a day or two, but in any case to get them you will have to click through to a recognizable merchant site.
  
  Then, publishers like Baen Books offer a large number of their e-books for free as loss-leaders. You'll notice that it's highly unusual for them to give you more than the first one or two books in a series, and the rest will cost you retail.
  
  Another clue to legitimacy is if the site offers payment in multiple currencies: there's almost no chance that any site accepting payment in US$, GBP and EUR is legitimate, because very few books are made available from a single distributor worldwide.
  
  All that said, if a site seems too good to be true, google that site's name and the names of some of the authors. Odds are they already know somebody's pirating their work, and they frequently rant about those sites on their own blogs.