I used to do that a year or two ago and it was pretty straightforward back than (there was «Export to epub» link or something like this on the left pane of a Wikipedia page). Not any more! Currently, there are two options for export in the book creator, namely, «pdf» and «plain text» (only «pdf» for single article).
What is worth I did not manage to find any clear info on why it is not there. The article at https://en.wikipedia.org/wiki/Wikipedia:Tools#PDF.2C_EPub.2C_Odt_and_LaTeX mentions it is still possible to produce epubs and re-directs to some weird page.
Do I miss anything obvious or they really disabled this useful functionality for good?