To elaborate a bit: choose the device that works best for you, for the task. You don't have to limit yourself to just one device. Nor do you have to carry every device everywhere.
A dedicated e-reader like a Kindle is the absolute best for sharp, readable text as well as long battery life. Most e-readers have backlighting that makes them usable in virtually all lighting conditions. And they're usually a little lighter and thinner than a good tablet. But they're also one-trick ponies that don't do much besides letting a user black-on-white text, so you probably won't want to carry one everywhere you'd carry your phone.. That said, they're _very good_ one-trick ponies, worth having if you do a lot of e-book reading.
My Galaxy S9 is about as large a phone as I can put up with; there are times when I wish it were smaller. There's no way I'm going to carry a phablet everywhere I have to carry a phone. While I suppose I could read an e-book on the S9, I would never choose to as long as I have access to larger screen.
When I expect to be killing time in a waiting room, coffee shop or departure lounge, I carry one of my tablets, usually the Fire HD 8-inch. (If the Fire's battery is nearly depleted, I also have a larger iPad 9.7-inch. Larger, nice screen, but less convenient to carry because of its size.) I consider the 7-inch, non-HD Fire tablet usable, but not optimal, for reading.
At home, I usually grab either the iPad or a "dead-tree edition". If I want to strain my eyes on tiny print, I'll read a software EULA -- not a phone screen. And I'm considering getting a Kindle as my bedtime-reading companion.