Magic the gathering general question about book burning like why is it so... cheap ?

The opponent gets the opportunity to choose the effect that is best for them. Lightning Bolt is relatively expensive because it's cheap (mana wise), effective and gives you options. With Book Burning you usually don't get the response you want from it. If you want them to take the damage they'll usually mill and if you want them to mill they will probably take the life loss. Book Burning it far to narrow in it's use whereas LB can be used for whatever you need it to do.