Let the initial number of books be X out of which A are biographies.
Given : A/X = 20% or 1/5 . Hence A = X/5
Let the new no of biographies added be Y.
Given (A+Y)/(X+Y) = 37.5% or 3/8 which gives Y = (7/25)X
Hence increase in biographies = Y/X = 7/5 or 140%