"Live in the now."
That is an utterly meaningless statement. It's akin to saying, "Exist in existence." Well, yeah. As if we had a choice. There's no place to exist but in existence.
Where else are I supposed to exist? Non-existence? That'd be impossible.
Equally impossible is for anything, naturally including we humans, to exist anywhere but in the present -- now. So save your money on books that claim to teach you how to live in the now.
You're already expert at this. There's no place to be except now.
But what about thoughts of the past and future? Those are taking place now. Do you really believe that you can time travel into the past or future? I hope not. There is no evidence that this is possible.
So if you consider that you can exist in the past or future, you're deluded.
Ah, but what about the Theory of Relativity? People or clocks traveling at different speeds experience time differently. And light doesn't experience time at all.
(Leaving aside the fact that almost certainly, light lacks the ability to experience anything.)
Doesn't matter. Someone traveling at a speed that approaches 186,000 miles per second, light-speed, still would be having that experience now, not in the past or future.
Yes, conscious beings in the space-time continuum can disagree about the order in which things happen. Past can be future, or the future can be past, depending on one's perspective according to the Theory of Relativity.
Still, my argument is that these perspectives are still occurring now, in the present moment experience of each observer.
Given my affection for mindfulness practices, I do consider that generally speaking it is good for us to be attending to what is physically present now, as opposed to thoughts of the past or future that are occurring now.
It's all now. We have no choice about that. But we can choose whether to be present to the world outside of us now, or present to the world within us now.
True, in one sense everything is within us, since conscious awareness resides in the brain, and the brain is inside our head.
What I'm talking about is what the focus of our consciousness is -- the present moment of the physical reality that surrounds us, or the present moment of our thoughts, emotions, and such.
So enjoy the fact that you already possess the power of now, the title of an Eckhart Tolle book. And you didn't have to pay any money to Amazon, or another bookstore, to enjoy this power.