File types and readers - maximum coverage

Question

Assume that an author is about to launch an ebook. In order of priority, what are the most widely-readable file types, in terms of actual users? In other words, to make the book available immediately to as many human beings on ereaders as possible, which files must the author produce?

My working assumption has been that .azw and .epub would cover the overwhelming majority of people using an ereader, but I am wondering if that is incorrect.

Please note:

• My question is not, "How many ereaders/platforms/software packages can read .xyz files?" It is, "How many people can read my book if I publish .xyz, .abc, and .whatever formats?"
• My question is specific to making the book widely available in ereader-friendly formats. Obviously, with a couple of ereader formats plus .pdf, .html, and .txt versions, one can get universal readability. I'm asking, what does one have to do to get near-universal readability in ereader-friendly forms that offer at least some formatting capabilities?

Edit: Answers should include statistics whenever possible. The goal is to go beyond the conventional wisdom and get into whether that wisdom is correct.

Answer 1

I think that if you provide .epub, .azw and .pdf versions of your book you can be pretty sure that you have covered the largest share of your potential readers.
For those that need another format, they can easily use Calibre to convert it, especially if they have access to the .epub version.

Edit: I don't have any statistics, but if you consider

• the formats offered by the majority of the online stores;
• the formats supported by the ereaders available on the market, and by the various PC or handheld devices ereading software;
• that it is possible to convert easily from one of the said common formats in to another less common one with Calibre, simply by using its easy and user-friendly interface;

you will have that the stores mainly offer .azw (or the rather equivalent .mobi), .epub and .pdf ebooks; that if you pick any random ereader or ereading software, it will be capable to read at least one of these formats, and often all of them; and that if for some reason someone needs another format, he can start from one of the said formats to get what he needs.
So even without any "official" study (assuming that any study in this respect can be considered meaningful of the entire market) that supports this empirical evaluation, you can be pretty sure that if you provide these three formats your book will be almost universally available; if I have understood your question correctly, you are asking how to reach the 99,99% of readers, and you should be able to do so with these three formats.