Berlin's Four Essays on liberty remains one of my most treasured moments (and books) in my intellectual journey. Lucid and intelligent writing, with none of the obscurantism. Of course, I have all Berlin's books.

A recent book and very good read I can recommend is Isaac & Isaiah by Caute which charts the rivalry between Isaiah Berlin and Isaac Deutcher (the author of an influential biography on Trotksy). I had not been aware of Berlin's (priviledged) background and the book helped contextualise his thinking.