The good news is that once you've got a text in one eBook format, it is relatively painless to publish it in additional formats (PDF being the notable exception, though it is generally easy to get a document into PDF format--you can do a decent job with a converter such as Calibre--see below--but translating the format properly is sometimes tricky).
At the very least, you should publish in PDF format. While not a particularly good format for reading on portable devices, it is a universal format that can be read on any any modern eReader, and, of course, on a PC screen.
I would also suggest formatting the book in the open EPUB format, and in a Kindle format (either .MOBI, AZW, or .KF8). I would then link to all of the formats on your web page, possibly with instructions on how to get the volume onto a particular device. You could also consider self-publishing the volume to the Kindle store, but bear in mind that there may be restrictions, and I believe you will be forced to charge for the volume. The iTunes Store is another possibility, but comes with its own restrictions and formatting / conversion guidelines.
You should definiltely download the fine E-Book manager, Calibre, which can convert between formats and is free.
In the end, there isn't one true standard for eBook formats yet, but it is not difficult having a few different formats on your web page.