Let the number of pages Susan read yesterday be \( x \). According to the information given:
\[ \frac{1}{3} x - 8 = 17 \]
To solve for \( x \), first add 8 to both sides:
\[ \frac{1}{3} x = 17 + 8 \] \[ \frac{1}{3} x = 25 \]
Now, multiply both sides by 3 to isolate \( x \):
\[ x = 25 \times 3 \] \[ x = 75 \]
Therefore, Susan read 75 pages yesterday.