Krishnamurthi’s teaching on “observer” / Observed i.e. Drk Drshya viveka struck me immensely in my early days, to understand the “conditioned mind” and transforming it to a “silent mind”.
I think the First and Last Freedom is one of his best and complete books, easier to read, and with very pertinent Q&A. The linked document contains introduction from the book and its list of contents.
Krishnamurti Talks and documentaries