the logarithm of quaternion

I can't see the page in Google Books, but what you apparently have there is the logarithm of a unit quaternion $\mathbf q$, which has scalar part $\cos(\theta)$ and vector part $\sin(\theta)\vec{n}$ where $\vec{n}$ is a unit vector.

Since the logarithm of an arbitrary quaternion $\mathbf q=(s,\;\;v)$ is defined as

$\ln \mathbf q=\left(\ln|\mathbf q|,\;\;\left(\frac1{\|v\|}\arccos\frac{s}{|q|}\right)v\right)$

where $|\mathbf q|$ is the norm of the quaternion and $\|v\|$ is the norm of the vector part (and note that the vector part of $\ln\mathbf q$ has a scalar multiplier); applying that formula to a unit quaternion yields a scalar part of $0$ (the logarithm of the norm of a unit quaternion is zero), and you should now be able to derive the formula for the vector part.

Just recall $\exp(\alpha i) = \cos \alpha + i \sin \alpha$ for complex numbers, the quaternion (remember a quaternion is just 3 complex numbers which all have the same real part) version is by direct analogue and take logarithm of both sides.