Is the book wrong? -- How to sketch frequency response obtained from H(z)? This question is directly related to digital signal processing, so I am asking here.
How to sketch frequency response obtained from $H(z)$?
I'm adding an example and its solution below. I did not understand some of the things for question 7.4, part D.
Any help is appreciated!

There are three points in graph. How and why the second from left is placed there? What's its purpose?
I have no idea how phase graph is created. What is the relation between x and y axis? What do they actually represent?
Why the book says &quot;give me function of frequency&quot; but the solution has $\omega/2\pi$ in x-axis?

Question 7.4:

An LTI system is described by the difference equation
$$y[n] = \frac{1}{3}(x[n]+x[n-1]+x[n-2]) $$
a. Determine the system function $H(z)$ for this system.
b. Plot
the poles and zeros of $H(z)$ in the z-plane.
c. From $H(z)$ obtain
an expression for $H(e^{j\hat{\omega}})$, the frequency response of
this system
d. Sketch the frequency response (magnitude and phase)
as function of frequency for $-\pi \le \hat{\omega} \le\pi$.
e.
What is the output if the input is
$$x[n]=4+\cos[0.25\pi(n-1)]-3\cos[(2\pi/3)n] $$

Solution:
As far as I can tell, the book is not wrong. (By the way, you're studying a 3-point moving average filter here, and it's useful to know about such filters.)

Your 1st question: That center dot on the Mag. and Phase curves serves no purpose that I can see. I suggest you not worry about that center dot.

Your 2nd question: The Phase curve is generated by evaluating the angle of the  $e^{-j\hat{\omega}}$ phase term, in the last line of the $H(\hat{\omega})$ expression in Part (c) of the Solution, as $\hat{\omega}$ goes from $-\pi$ to +$\pi$. As $\hat{\omega}$ goes from $-\pi$ to +$\pi$, the Solution's $\hat{\omega}/2\pi$ Phase curve freq axis label goes from -0.5 to +0.5.

(Note: as you study the Solution know that the $H(z)$ in Part (c) is not the $H(z)$ in Part (b). The $H(z)$ in Part (c) is the $H(z)$ from the Solution's Part (a).) 

Your 3rd question: It's a good question. In the literature of DSP there are a number of different notations (different symbols with different dimensions) used to represent frequency. And those various notations certainly can be confusing at first. mmswe, have a look at the following web page, it might help you with your 3rd question:

http://www.dsprelated.com/showarticle/199.php