You should be able navigate to the book in the sample distribution, then look for a view "Book EPUB" and download the .epub file. Then open in an EPUB reader. (Or, in Firefox you can open a new tab, instead of doing the download).
The EPUB navigation pane works, but the internal Pier book links (in the table of contents, say) do not work. I'll look into it.
Great I've seen my first Pier ePUB book. I could view it in the Firefox add-on.
Have you had any luck viewing the output ePUB file with iBooks on iOS? I tried with my iPhone but it wouldn't open. I also noticed that the section link wasn't numbered - any thoughts?