There is relatively little current fiction available for free as ebooks. (Enough is available to read for a long, long time, but that's still a tiny fraction of books published.)
There's the
Baen free library of science fiction books. Feedbooks has a collection of
recent, rather than classic-public-domain, books submitted by their authors. Harlequin's
17 free sample romances are still available. Everything by
Cory Doctorow is released under Creative Commons, in several formats. And many publishers release occasional free ebooks as promotions.
But overall, for most books in print right now, there are no legal sources of free ebook versions.