Dear Dr. Sacks . . . You asked me if I could imagine what the world would look like when viewed with two eyes. I told you that I thought I could . . . But, I was wrong.

When Susan Barry first wrote to Oliver Sacks, she never expected a response, let alone the deep friendship that blossomed over ten years of letters.

Sue, herself a neuroscientist, wrote to share an extraordinary development in her own medical history. Born with problems with her vision, Sue had been told she would never acquire the ability to see in 3D - and yet she did, a development at odds with decades of research. Within days, Oliver replied, Your letter fills me with amazement and admiration.

Sharing an interest in visual perception and a deep love of science, Sue and Oliver began writing back and forth, delving deeper into the mysteries of sight and marvelling at the adaptive capacity of the human body.

But in a painful twist of fate, as Sue's vision improved, Oliver's declined, and his characteristic typed letters shifted to handwritten ones. Sue later recognised this to be an early sign of the cancer that ultimately ended his extraordinary life.

A funny, fascinating, and intimate glimpse of the great Oliver Sacks, Dear Oliver is also a love letter to scientific inquiry, and a testimony to the power of friendship at any time in life.