Quantization of electromagnetic field Every quantum optics book starts with quantization of electromagnetic field. Why?
My understanding:
The quantized electric field consist of photon and it helps to derive some properties of photon.
Is that so?
I need clear explanation.
 A: 
The quantized electric field consist of photon and it helps to derive some properties of photon.
Is that so?

No, it is the electromagnetic field, the classical wave solution of Maxwell's equations that can be shown to consist of photons.
Electric fields and magnetic fields can be shown as far as quantum theory goes to be connected with virtual photons, but one has to learn quantum field theory for this.
See this simple one photon at a time class experiment, where the accumulation of photons shows the interference pattern of light. This can be shown  mahematically , but it needs a  quantum field theory background.

Every quantum optics book starts with quantization of electromagnetic field.

Because quantization is inherent in discussing "quantum", so "quantum optics" has to do with quantization of the electromagnetic field by definition of the field.
A: Electromagnetic field is one of QM systems. QM systems differ from classical ones in a superposition principle and the ways of calculations of probabilities.
A: It seems to me that you are in a slight confusion regarding the meaning of "quantization of the electromagnetic field" and "photon". Let me try to explain my understanding of these concepts. (I know that most of this will be familiar to you, but perhaps things will become clear when they are spelled out.)
1. Quantization of the elctromagnetic field
Generally speaking, quantization means to interpret a (classical) physical system as a quantum physical one. To do so, one associates observable quantitites of the system with hermitian operators on some Hilbert space. In the quantum theory of the electromagnetic field, these quantities are the electric and magnetic fields. If one chooses Coulomb gauge, they can be expressed in terms of the vector potential, a single field. This field represents a continuum of observables, namely one for each point of space. To faciliate quantization, the theory is constrained to a closed box, in which the field can be expanded in a countable number of field modes. In a classical theory, it is then described by a countable number of quantities, namely the amplitudes of each field mode. In the quantization of the EM field, these numbers are promoted to operators, yielding creation and annihilation operators. The study of these operators leads to the notion of photons.
2. Photons
On inspecting the Hamiltonion of the quantized EM field, one sees that it is has the form
$$H=\sum_\text{j} \hbar \omega_j \left( \hat{a}_j \hat{a}_j^\dagger + \frac{1}{2}\right)$$
where the summation goes over all modes. In order to find the energy eigenstates, the eigenvalue problem of the operators $\hat{a}_j \hat{a}_j^\dagger$ has to be solved. Using the commutation relation
$$\left[ \hat{a}_j, \hat{a}_j^\dagger \right]$$
one can show that each mode has the eigenvalues $n=0, 1, 2, \dots$. The corresponding number eigenstates $|n\angle$ are interpreted as states in which n quanta of energy are present in the field mode considered - and a quantum of energy in an optical field mode is called a "photon".
