The quickest way I've found to check the time during reading is to press the HOME and then MENU buttons. A Clock then appears at the top left.
Then you can select your book again to resume reading.
I guess that's pretty quick.
I would like to see the time and battery displayed at the bottom of the screen next to the page numbers, but I suspect it would require page refreshes (flashing page) every minute so they didn't add this.
--By the way, if you haven't discovered it already, when you're on your home page, you can press the top right corner of the screen to quickly turn off/on wifi only when you need it, to save battery power.--