If you analyze the market, which format should you publish your eBook in? Some people will publish in their favorite format. Some people want to publish in the most popular format, and some people will publish in all formats. Here's a bit more information to help you compare the formats and decide.
My husband and I did a lot of googling to look for data. (I asked my husband to help because he's a better googler.) What I wanted to find was a "market comparison (pie chart) of sales of eBooks in 2010 by format." I didn't exactly find that. I found some other interesting data, and from that, I can draw some inferences.
Aptara Corp is a big company that takes people's data and text, makes it pretty, and publishes it. The company bills itself as a digital publishing solution. For the past three years, Aptara has surveyed publishers. Aptara published the results of this year's eBook market comparison survey. If you read this survey, remember to take into account who filled out the survey. The people taking the survey worked in the publishing industry.
Another interesting set of data I found was published at beatanews.com. It talks about the Worldwide eReader Shipments (so, hardware sales) for the third quarter of 2010. Instead of the eBook format, this survey compares the eReaders sold.
We found several other articles, but those are the most interesting. What I've gleaned from those articles follows.
Amazon is selling the most hardware.
Amazon is selling the most dedicated eReaders. Amazon has about as much market as Pandigital, Barnes and Noble, Sony, and Hanvon combined. (This is my chart made of the data betanews published in their article.)
The iPad and Kindle are close to tied for reading preference.
This is a chart I made off of the data in Aptara's survey results.
The biggest sellers are Amazon and the publishers themselves.
The Business Insider website indicates that Amazon sells 2 ebooks for every printed book and that Amazon has about 50% of the eBook market. The results of Aptara's survey agree that Amazon makes the most sales, but give different numbers. I made this chart from data in Aptara's survey.
What I've pieced together
None of those figures quite give the whole picture. For instance, iPhones are mighty popular, and you can read just about any eBook on an iPhone. On my iPhone, I use my kindle software a lot, but I also use Stanza, iBooks, and eReader. The app I use just depends on what format I've downloaded. Business Insider claims that most books read on the iPad are actually read on the Kindle software. I don't know where they got that piece of datum, but it goes along with my reading habits.
Another consideration is that if it works on an iPhone, it works on an iPad.
And yet another consideration is that Amazon just started selling eReaders for $79.
My recommendation then is
I think the eBook should definitely be made into the Kindle's format, whether or not you actually want to sell it on amazon. (You can let readers download the file and they can transfer it to their kindle without using amazon's whispernet.)
If you have worksheets that people might want to print, make them available on your website as a pdf and make the links obvious. The easiest way to copy a page from a kindle is to photocopy it, and if you're selling worksheets, you might want to make it easier on your readers.
If you are giving away an eBook to get people to sign up for your mailing list, give them a choice between a pdf and something else, even if the something else is just a text file. PDFs are difficult to read on any reader other than the iPad.