In my book, energy stored in a stretched spring or work done in stretching a spring is given as E=0.5ke²

On another page, it is given "you can calculate the energy stored per unit volume as energy=0.5×stress×strain"

Both seem same to me. Is it so?

They are related, but not exactly the same. Note that the units of the first equation is energy, which is measured in Newton-meters, or N-m. The second equation is in units of energy per unit volume, or N-m/m^3. So to calculate total potential energy given stress and strain you must multiply by the volume of the object (area times length).

Understood. Thanks!