Losing the ability to focus on books is usually a setup problem before it's an attention problem: a phone within reach turns every paragraph into a choice.
Your first day
Read for fifteen minutes with your phone in another room, not just face-down nearby. Pick a book already sitting around, ideally under three hundred pages. Set a timer, on your phone before you move it or on a kitchen timer, and read at a normal pace until it goes off.
Most people leave the phone face-down on the same table, thinking that's enough. It isn't. Studies suggest even a silent phone nearby can pull attention, though how much is debated. The practical fix is physical distance, since willpower is a poor tool against something built to get your attention.
The thing most people miss
Moving the phone to another room, not just out of sight, helps many people, and a week of trying it will show whether it helps you. The belief that your attention span has permanently changed may be wrong for many people. What changed is what's within reach.
Once the phone is physically away before you sit down, the reading itself stops being the hard part, and you don't need willpower for it.
What not to do
Don't push through the moment you catch yourself rereading the same paragraph twice. That's not a sign to concentrate harder, it's the signal to take a short break. Another few pages forced past that point rarely sticks anyway, and it just trains you to associate reading with fighting your own attention.
You'll know it's working when you look up and realise you haven't checked the time. Reading two books at once, one fiction and one nonfiction, is worth trying next, since for a lot of people that keeps either one from stalling out half-finished.