Twice the age of 6 is 6 x 2 = <<6*2=12>>12 years old.
At 12 years old, Mandy was reading books 5 x 8 = <<5*8=40>>40 pages long.
8 years later, Mandy was 12 + 8 = <<12+8=20>>20 years old.
At 20 years old, Mandy was reading books 3 x 40 = <<3*40=120>>120 pages long.
Presently, she reads books that are 120 x 4 = <<120*4=480>>480 pages long.
#### 480