They say you should never learn how the metaphorical sausage is made, but I do! I do want to learn how the sausage is made.
Sher's diaries are a great read, even if you don't love all of his performances (I was not blown away by his Falstaff or his Lear, to be perfectly honest). He was such a calculating, deliberate, and technically-minded actor, and it's fascinating to follow how he builds a character, line by line, and sometimes eyebrow by eyebrow. His diaries and his insights into himself and his craft are a gift to all self-respecting Shakespeare geeks.