Let \( x \) be the number of pages Susan read yesterday. According to the problem, we know that:
\[ \text{Pages read today} = \frac{1}{3} \text{Pages read yesterday} - 8 \]
Since she read 17 pages today, we can set up the equation:
\[ 17 = \frac{1}{3}x - 8 \]
To isolate \( \frac{1}{3}x \), we first add 8 to both sides:
\[ 17 + 8 = \frac{1}{3}x \]
This simplifies to:
\[ 25 = \frac{1}{3}x \]
Next, to solve for \( x \), we multiply both sides by 3:
\[ x = 25 \times 3 \]
Calculating this gives us:
\[ x = 75 \]
Thus, Susan read \( \boxed{75} \) pages yesterday.