The obituary of Powell published by the Royal Society:

https://royalsocietypublishing.org/doi/full/10.1098/rsbm.201...

https://www.damtp.cam.ac.uk/user/na/NA_papers/NA2017_04.pdf

by Optimization Methods and Software:

https://www.tandfonline.com/doi/full/10.1080/10556788.2015.1...

https://www.mathopt.org/Optima-Issues/optima99.pdf

and by the SIAM Activity Group on Optimization:

http://wiki.siam.org/siag-op/images/siag-op/6/64/ViewsAndNew...