For many years, I've been drawn to the ancient classic Chinese work: Tao Te Ching..the title translates as "The Way of Life"

You'll find many ideas about being "flexible" in life (like water) to whatever happens to you in that work, and the many derivations of it.

Human beings basically have two modes of operation. Firstly, a "limited" mode (physical-reality-focused) and an "unlimited" mode (non-physically-focused).

The "limited" mode provides focus, and therefore stability and power, to life but as soon as you focus on something specific, you automatically exclude (or limit) the other things you are not focused upon. (See Why have we accepted limitations on our existence?)

The "unlimited" mode provides a sense of connection to All That Is (The Universe) - it reminds us that we are multi-dimensional beings, and it gives us access to an infinite storehouse of information (through "intuition" and "inspiration"). All the truly great works of art (for example) are considered to be so great because we sense they are expressing physically this sense of unlimitedness.

See So What Is The Now? for some further information on the difference between these modes of operation.

The concept of "Flow" is basically the same idea as operating in this "unlimited" mode. It is exactly the same concept as being "in-the-present-moment".

So any books or philosophies that guide you into being more "present" are also guiding you to being in that "flow" state. Many of the Eastern traditions (Taoism, Buddhism etc) aim to get you there and perhaps works like Eckhart Tolle's The Power of Now, from what others have said, might also be guiding you towards there (I haven't read that book yet myself).

One thing to bear in mind about being "present" (or in the "flow") is that you'll never be there all the time.

That's not the nature of the illusion of 3D physical reality. After all, if you were in the "flow" state all the time, how would you then know what the "flow" state was? :) It's like if you didn't know what darkness is, how would you know what light is?

But just to complicate things further, there are those who are saying we are now (within the last several months) no longer existing in the illusion of 3D physical reality but in a 4D physical reality where we are projecting the illusion of being in the illusion of a 3D physical reality...so perhaps we now have access to more flexibility regarding our interpretation of the rules of life, like whether it is possible to be more in a "flow" state, than ever before...but that's another story :)