Andrew Hoyle/CNET

BERLIN -- What do you get for the phone that has everything? The Samsung Galaxy S4 is possibly the best smartphone ever made, but one thing it doesn't have is an e-ink screen for reading the latest page-turner.

That's where the PocketBook S4 case, unveiled at technology bunfest IFA 2013, comes in. It's a case for the Galaxy S4 that contains a 4.3-inch e-ink screen.

But my S4 already has a glorious 5-inch high-definition full-colour screen! Why do I need a spare screen that only does one colour hanging off the side of my fancy, high-class phone?

Let me tell you, dear reader. Unlike an electronic screen, you can stare at an e-ink display for ages without getting eyestrain. You don't have to worry about glare from the sun or bright lights reflecting off the glass screen, and, best of all, an e-ink display uses next to no power compared with a battery-guzzling bright electronic screen like the dazzler on the S4.

So if you want to catch up with a marathon session of "Fifty Shades of Grey" without caning your phone's battery or knackering your eyes, the PocketBook could be worth a look. Made by e-book-builder PocketBook, the case talks to an app on the phone, where you store your e-books. You take an e-book stored on your phone and zap it to the e-ink screen, turning pages with the volume key.

No price or release date has been confirmed.