I understand what you're saying about at least including some wording and I see that many posters like e-books, even exclusively, but this is my reasoning to even keep a mention of e-books out of it -
The way I see it, if 60% of voters prefer an e-book, then one will always win.
"Literary" itself is already a restrictive category and after that I'd rather keep the pool as wide as possible. Also, this isn't just about the winner, it's also about people discovering other books just through nominees, even ones that don't get fully nominated.
So, to me, putting in even a mention of e-books in the wording may make some hesitant to nominate certain works. Those works may not win, but they'll then be exposed to everyone reading the thread and some may decide to look into and possibly read that work anyway.
I see nominations not only as a process to a monthly selection but also as a discovery of possibilities.
But, even if a non-ebook wins, why is that then bad? It would mean that that month, more people preferred and voted for a non-ebook. Anyone who doesn't want to read a non-ebook can always read another ebook nominee or wait and join again the next month.
I don't want to exclude anyone who only reads ebooks, but nor do I want to exclude the realm of non-ebook possibilities. So I think leaving it with no stated preference to any format is the best solution for everyone because e-books will still often win, but non-ebooks can still be nominated and discovered without hesitancy, and possibly even win now and then, if that's what the voters prefer.