title_body: Where can I download free ebooks?
upvoted_answer: I am not sure what you consider an interesting book.  That depends much on the reader. Anyway, I edited your question to remove the reference to "interesting books", since it is a matter of opinion and as such is generally a proscribed issue on StackExchange.
  
  Asking for, or giving a list of places where you can find free ebooks is also proscribed, as it can give rise to endless lists and answers. Furthermore, it would make little sense since the question would become far too broad (another proscribed situation), because you did not specify (fortunately, I think) the kind of books you are interested in.  I am saying fortunately, because it does leave open the one proper option, which is to describe in general terms some current disseminations and pricing practices, which structure the ebook market.
  These should be hints as to how to search on your own the current market for sources of free book. Ad searching is good, because its is always an open door for serendipity.
  
  Answers to your question may also depend on how legal you expect the download
  to be. For several reasons I would discourage downloading from illegal
  sites and I will thus ignore this possibility.
  
  Parameters that may play an important role include :
  
  
  the kind of book you are looking for : romance, scifi, thriller,
  philosophy,   mystery, comics, etc.;    
  the book format:  epub, mobi,
  pdf, scanned pdf, ... knowing that  readers do not always accept all
  formats;
  your tolerance to DRM  (Digital Right Management systems).
  Though rarer for free books, they manage to be a pervasive annoyance
  in any context;
  whether you might be willing to pay a very small
  price rather than nothing.
  
  
  Note also that some books (whether free or not) may not be
  downloadable from some countries.
  
  Without attempting to list sources, 2 sites are major entry points for public domain books, that cannot be ignored:
  
  
  The primary source of well formatted free ebooks is certainly
  Project Gutenberg. I would say they have a very large
  collection of interesting books, well digitalized.
  If you are not fussy about format, there is also the Internet
  Archive, which has pointers to lots of library resources, some
  of which do offer public domain books for free. And there is the
  Internet Archive own repository. But these books may be in
  awkward formats, or poorly digitalized ... the technical quality is
  very variable.
  
  
  But since the above sources are making available public domain books, they are   unlikely to contain recent books.
  
  Many independent authors give some of their ebooks for free, though
  you seldom get complete series (but it does happen). They will often
  ask you to subscribe to their mailing list in exchange for the free
  book(s). And some of these mails will contain information about other
  free resources. See BookFunnel and Instafreebie. Many author will give you books for free, in exchange for honest independent reviews on various sites. You may also get pre-release versions (ARCs, i.e., advance review copy) in exchange for reviewing or proof-reading the book.
  
  Most retailers (including Kobo, Smashwords or Amazon) have a lot of
  books available for free, sometimes permanently, sometimes only for 2
  or 3 days. Grab what you want, as it may be gone tomorrow.
  
  You can subscribe to various daily mails that will keep you informed
  of such opportunities. Examples include BookBub, The Fussy Librarian,
  Book Cave , Robin Reads, BookBarbarian and many others.  Some mailing
  lists may specialise on topic, publisher or retailer. Just subscribe
  to some.
  
  Some publishers give away free books to people who are subscribed to
  their mailing list. Some of them are quite good, though they are often
  the first volume of a series. For example, the publisher Arc Manor (aka Phoenix Pick) specialises in science
  fiction, and has one book for free every month.
  
  Many books can be bought for very low prices, such as $1 or $2,
  including award winners and classics.
  
  A good source can also be to buy book bundles. Humble Bundles has some
  great opportunities in all kinds of books (including comics). You can
  typically get a bundle of 25 books of good average quality (including
  some top ones) for $15. You may also want to look at StoryBundle
  bundles, though they are less varied, and a bit more expensive. Besides these two bundling retailers, you can sometimes get bundles from publishers or retailers.
  
  There is a lot more to be said and found, but this ought to get you
  started. My information is probably biased by the kind of books I am
  interested in.
  
  But it is also important to help by paying when you can. Most of these books
  have to be produced and edited by people who try to make a living.