Oaxis has been selling secondary e-ink screens for Android and iOS phones for a number of years. The screens are built into a dedicated case, which protects your device from the rigors of daily life. Oaxis released the InkCase i7 a few months ago for the iPhone 7 and this is likely their best product yet.
The InkCase i7 features a 4.2 inch e-ink screen with a resolution of 480 by 800 and 217 PPI. This device hooks up to your iPhone via Bluetooth and is good for about a month before you have to hook it up to the proprietary magnetic charger.
It is important to note that the e-ink screen does not mirror what is displayed on your iPhone. You need to download dedicated apps the Apple App Store, including one that will read e-books and another that will display your photos as a slideshow.