The epub link on that page sends me to an online reader, from which the 'Download' button does precisely nothing. This might be due to all the browser-armour I'm wearing, might be a Firefox thing, don't know. The PDF link works fine.

Anyway, if this is happening to anyone else, just copy the link for the epub button on the main page, and add "?download=true" to the URL (mirroring the pdf link). You will then get an epub file downloaded directly by your browser.