To find the total amount Miles spent on the three books, we need to add the prices together:
\[ 13.89 + 6.70 + 3.05 \]
Let's do the addition step-by-step:
- First, add $13.89 and $6.70:
\[ 13.89 + 6.70 = 20.59 \]
- Next, add the result to $3.05:
\[ 20.59 + 3.05 = 23.64 \]
Thus, the total amount Miles spent on the books is $23.64.
So the correct answer is:
$23.64