title_body: eBook format in nowadays: EPUB is de facto standard? It is a basic and old question...
  
  There was a "format war" for ebooks (a "Tower of eBabel" for users)... EPUB won?
  Wikipedia says that &nbsp; «... EPUB format is the most widely supported vendor-independent XML-based (...) is supported by the largest number of e-Readers (...). The popularity of Amazon.com's Kindle devices in United States has led also to the prominence of KF8 and AZW » ... It is a truth for this "Ebook  publishers and readers" community?
  
  So, putting in other words:
  
  
  EPUB is a world-wide consensus? It is the most popular, for ebook readers today? We can put all money in this format?  
  And about the "second place", what is the format? (PDF? MOBI?) This second format is still important for ebook-readers?  There are reliable and updated statistics? (2010 example, 2009 example)
  
  
  
  
  NOTES (EDIT)
  
  The question is about "Comercial + Public Domain" content.
  
  For public libraries and public domain content the perception statistics are  changing... People feel that there are "some standard" (EPUB?). 
  
  What we have heard (since 2009) in the comercial scenario is «PDF is still the format of choice for most people, though EPUB is getting respectable usage, with Mobi in third», but no reliable statistics about it.
  
  To join "public domain scenario" with "comercial scenario" perhaps another kind of statistics must be also used: a survey like "ebook users read more comercial or public content?"
  
  The world-wide statistics have distinct profile than USA-statistics, but most of the compiled data came from USA.
  
  About replacement of the old and stiff PDF by EPUB, we have heard that "the market" is waiting the advent of HTML5 + CSS3-paged completion (be terminated and official W3C recommendations) and consolidation... How many years?
  
  About reference numbers: how many ebook-titles user have (at Internet) in each format and each scenario?
  
  
  Public domain and open content:
  
  
  PDF: a lot of titles (!!) ... some reliable statistics?
  EPUB: as of March 2014, Project Gutenberg claimed over 45,000 items in its collection (but have no format-download statiscs); SciELO and many other minor public domain e-book projects are also distributing ebooks. Another source of Ebooks, scientific articles: PubMed Central have 3.1 MILLION articles that now (2014) are ditributed also in EPUB format.
  
  Comercial universe: ...? Amazon dominates?
upvoted_answer: First, a note: I will not be considering PDFs in this answer, because I do not consider PDFs to be ebooks. There are a number of reasons for this, like lack of reflowability and end-user customization options, but the primary reason is that PDFs are not generally sold at sites that sell ebooks--you can't sell your PDFs through iBooks or the Kindle store or whatnot. Others have other opinions, and that's fine&mdash;the relevant info is that I won't be considering PDFs here.
  
  In terms of variety of devices supporting the format, epub is the clear standard&mdash;it's supported on nearly everything that reads ebooks. When you look at numbers, though, it's a different story: according to an article at Digital Book World, a recent Book Industry Study Group found the following statistics to the question "Where do you typically acquire ebooks?":
  
  Amazon.com website: 51.3%
  Amazon App: 15.7%
  iBooks/iTunes (Apple): 8.2%
  Barnes &amp; Noble App: 6.1%
  Barnes &amp; Noble website: 5.7%
  All other sources: 12.8%
  
  
  This means that Amazon accounts for roughly 2/3 of the sales of ebooks, making their proprietary mobi/KF8 format the dominant format.
  
  In essence, there is no "winner" yet, and there is not likely to be any time soon. Apple, Google, Barnes &amp; Noble, and Kobo are enough to ensure that epub will remain alive and kicking for quite some time. With a majority of the sales, Amazon has no impetus to switch to a standard, and they don't appear to be in any danger of losing much market share. Where we are is where we're likely to be for quite a while, which means making two formats: epub and mobi/KF8. fortunately, Amazon has gotten much better at making tools that convert from epub to their format, so concentrating on epub will give you something for Amazon as well with little extra work.