Elastic potential energy is given by formula
here we know that
Now using above formula we have
So elastic potential energy in the chord is 14400 J
The only thing in either one of those formulas that we don't know is the mass of the book.
The transit method requires watching the light output of a star over long periods of time. A transit occurs when the planet crosses in front of its star from earths point of view. Since there is a small object (the planet) now blocking some of the star, it appears to dim a little bit for a while until the planet passes. If we are in a position where that occurs regularly (most paths of planets do not happen to be on the line of sight between earth and their star) we can deduce the period of orbit. From the amount of dimming and the period you can estimate the mass