When the iPad came out I was able to convince myself that it really is my responsibility as a publisher of a digital book that I should have this device for testing.
Ha! Yeah right, I just thought they were cool and wanted one. So the big question is...
Can you read Digging Into WordPress on the iPad?
To which I say:
I tried a couple of different PDF readers, and so far I've found GoodReader to be the best. I'm not 100% in love with it, but it's pretty decent. There are a bunch of different ways to get files into it. I found the best is to connect it your your Dropbox account and yank the files from there. Dropbox is free and wicked awesome, so you might as well have an account there anyway. But if you don't want to, you can get files onto it by creating a local network and transferring the files via the web. It's kinda weird though, fair warning. Other PDF readers could do it through iTunes, but that was almost just as weird.
So I'm a Kindle owner as well, and I tried numerous things for trying to get Digging Into WordPress onto it. You can read PDFs on the kindle. One way send the PDF as an email attachment to your special kindle email address and (for a small fee, like a few cents) Amazon will convert it for you and send it to your Kindle. Our PDF is very much not formatted to be ready for the Kindle screen, so the end result is rather unusable.
There also is an application called Calibre (cross-platform), which can connect to your Kindle and send stuff to it. You can put the PDF in there and "Convert" it first. I found the end result rather unusable as well.
So far, no great "automatic" solution for Kindle.
Most other eReaders (i.e. other than the Kindle and including the iPad) use the ePub format. We created the book in Adobe InDesign, which does allow us to export it into ePub format, but it's not a set it and forget it process. The end result of a straight conversion is pretty ugly.
So... we don't currently have an ePub version and we aren't currently working on it. If there was enough demand, we possibly could. It would likely be free to current buyers or a small upgrade fee just since it'll be a bunch of work on our part to get it done and it wasn't something we planned on or promised.
Why is it so much work? Well we put so much loving care into a custom layout! =). Our images don't just sit like blocks in the text (like in this blog post). They are custom positioned with text wrapping around them and many of them have captions. There is text in the margins that explains little things. There are popout boxes which focus on some special topic. All those things mean that exporting the thing to pretty much straight text means things end up in a strange order with text that awkwardly jumps around to different topics. Converting to the ePub format would be a manual process to make sure everything all makes sense.
Long story short
We have a nice looking PDF. A good eReader should be able to display PDFs well. Currently that's only the iPad.