For more about Powell, who was a towering figure in optimization theory, see this site.
Section 6.4 of the book "Numerical Methods in Scientific Computing, Volume I", by Germund Dahlquist and Å. Björck, contains a nice discussion of methods for finding the minimum of a function.
The book is very nice overall, in fact. If you are on a machine at OU, you can read a copy of this book online through Bizzell Library.