Why does an increase of airspeed result in a decrease of angle of attack?
On page 2-10 under the heading "Induced Flow (Downwash)", the book is making the point that if you have a wing moving forward horizontally at some fixed speed relative to the ground and some fixed positive pitch attitude relative to the horizon, and you surround the wing with a descending airmass, the wing's angle-of-attack will be less than if the airmass were not descending.  A rotor blade of a helicopter can be viewed as such a wing, at least at a given instant in time.   

The book is using the term "induced flow velocity" specifically to refer to the downward velocity of the airmass caused (induced) by the rotor blades.  I.e., downwash.   

The exact sentence you quoted was under the subheading "Out of Ground Effect (OGE)".  Since the ground is no longer acting like a "floor" to impede the downflow of air through the rotor system, you have more downflow than you would in ground effect.  

If you were hovering in ground effect and the ground suddenly disappeared, the downflow velocity would increase and the angle-of-attack of rotor blades would decrease.