Do I get a list of my ibook purchasers?

Apple does not provide that information. Authors are on their own if they wish to find out who their users are. Be sure to publish contact information in each book.

No. Like KYT says. You can have your email address on the publisher / copyright page and people may email you. - Fabe

How do I notify purchasers that there might be an update to the book?

Since there is currently no "built-in" automated process for doing this, you might try adding a "Check for Updates" interactive widget.

BASIC STRATEGY:

Each update of your book would include a widget that has two parts. The first confirms the book revision in which the widget is installed and the second polls a specific URL containing the identification of the most current update. Thus, the reader merely has to click the widget to check for an update. If the current and most recent revisions are both the same, then no update has been posted. If there has been an update, then the user has the option of downloading the updated version manually.

When the publisher releases an update, he merely updates the widget to be included in the new volume as described above and posts an update to the URL reflecting the latest revision is now available. I initially tested this using HTML 5 animation to indicate an available update. Thereafter, I tried the more simple approach of using JPEG images to reflect changes. And, if the 3.82 GB movie I'm trying to upload ever finishes, will go back and test the simplest method of using just a textual update. So far, the fist two work flows have worked flawlessly in the iBooks Author preview test mode.

This widget sounds like a solution, but I know nothing of building widgets. Does this thing exist somewhere (for sale?) or can you point me in a direction to learn?

I use an app called Hype (which is primarily a GUI driven HTML 5 animation application) to create my widgets. Basically I've had a copy of the app for a year or so but never used it. Then, last week I decided to play around with iBooks Author. At about the same time, Tumult announced an update to Hype that included the ability to export widgets for iBooks Author. So I figured this would be a good time to learn something about both apps and how well they might complement each other. In any case, here is a description of what I did.

1) To create the interactive portion of the widget, you open a Hype project and create a text box or insert a graphic or create an animation that identifies the "latest" edition/revision of your book on a appropriately sized background "scene". As a beginner, a text box or the pasting of a graphic image made in Photoshop (or a similar graphic application) might be the easiest thing to do. When satisfied, simply export your project as HTML 5 to a local folder. Now upload the folder to some online server space you have available.

2) Next, you create a second project. It will be much like your first in that you can create a text box, insert an image, or create an animation which identifies the current edition/revision of the book into which you will insert the widget. (Normally this will be the same as the previous project if this is your first edition.) In any case, to this project you then add an "iframe" widget box. Since I knew nothing about how to us the box, I went to the Tumult web site and downloaded a sample "iframe" folder to study. As it turned out, this sample project contained all the information I needed to complete my project and I merely coplied the sample HTML code as depicted here:

<iframe width="300px" height="400px" src="hummingbird.html" style="border:0px" scrolling="no"></iframe>

to the "iframe" box in my second project and adjusted the width, height, and source information as needed. After running a few preview tests to ensure the second project was running correctly, I exported the projects as a widget and added it to my iBooks Author test project for testing. When you create your next update, you update your second Hype project in order to include the new widget in your updated volume. Then, after the updated volume is released, you update the first Hype project and replace your original online HTML folder with the new one to alert previous purchasers of your update.

And that was all there was to it. Hope it helped...