Act 5 Perhaps Prospero's most decisive action in this act is to renounce his magic and drown his books. Why would he do it?

I think that Prospero renounces his magical powers for a
variety of reasons.  Here are some possible reasons why he would do
this:

1. He no longer needs his magic.  For most of
   this play, he has wanted to use his magic to punish those who did him wrong and to get a
   better life for himself and Miranda.  Now this is no longer
   necessary.


2. Perhaps he feels that the power that the magic
   has given him has corrupted him.  I think that he may feel that using magic to
   essentially enslave spirits like Ariel (as well as Caliban) is something that he should
   not continue to do.