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 «... 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)
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?