The equilibrium constant is
$$K=\frac{a_{\mathrm{H_3O^+}}a_{\mathrm{OH^-}}}{(a_{\mathrm{H_2O}})^2}$$
where ##a_J## denotes the activity of a solute ##\text{J}## in a dilute solution, ##\mathrm{[J]}/c^\circ## with ##c^\circ=1\mathrm{mol\cdot L^{-1}}##.
The book I am following says that the...