As jolhoeft remarked, sealing the e-reader in a plastic bag will do in a pinch. You can't (comfortably) do this with a bound volume. How exactly is this a point in favor of dead trees?

I worry less about water damage to my Kindle than I worry about water damage to physical books, even without any sort of protective covering. If I spill coffee on my Kindle, I simply wipe it off. If I spill coffee on a bound volume of paper, I consider it to be "ruined." My Kindle has demonstrated far greater fortitude than paper in weathering spills.

Put it in a freezer grade zip-lock bag. Works fine with buttons to turn pages. Not as sure about touch screens.