Let the number of pages Susan read yesterday be x.
One third of the number of pages Susan read yesterday is x/3.
Susan read 8 pages fewer than 1/3 of the number of pages she read yesterday today, so she read x/3-8 pages.
Since Susan read 17 pages today, the number of pages Susan read yesterday is x=3*(x/3-8).
Multiplying through the parentheses, we get x=3*x/3-3*8=3*x/3-24.
Combining like terms, we get 3*x/3-x=24.
Subtracting x from both sides, we get x/3=24.
Multiplying through the equation by 3, we get x=3*24=<<3*24=72>>72. Answer: \boxed{72}.