What does equilibrium constant mean?
I am sure the book, if it is any decent, should have said something like this already. $K'$ is not a measure of the forward reaction, nor is it a measure of the reverse reaction. It is a measure of the equilibrium position.

Equilibrium occurs when the rate of the forward reaction is equal to the rate of the backward reaction. By the laws of thermodynamics, this occurs at one special point where the instantaneous concentrations of all the compounds satisfy the equation

$$\frac{[\ce{CaO}][\ce{CO2}]}{[\ce{CaCO3}]} = K'$$

$K'$ for a reaction is a constant. Clearly, a large $K'$ indicates that when equilibrium is reached, there is a lot of $\ce{CaO}$ and $\ce{CO2}$, and only a little bit of $\ce{CaCO3}$. We say that the equilibrium position lies to the right. Conversely, a small $K'$ means that the equilibrium position lies to the left, i.e. reactants predominate.



The equilibrium constant is properly defined in terms of activities of the compounds, not concentrations. This means that for the reaction you gave,

$$K = \frac{(a_{\ce{CaO}})(a_{\ce{CO2}})}{a_{\ce{CaCO3}}}$$

The activity of a pure solid is defined to be equal to $1$, which means that you can simplify this by removing all the terms corresponding to pure solids:

$$K = a_{\ce{CO2}}$$

The "equilibrium constant" that you are learning that involves concentrations is a simplification.