Wonderful. What a treat: Bair's at getting to meet and write about these two giants and yours at reviewing "an uncorrected advance copy of the book".
It is always a challenge for anyone intellectually oriented to meet and deal with people of genius. Most of them are quite arrogant and inaccessible and can make the average folk feel unqualified for the task of even having a casual conversation with them at a book reading session.
But, I guess they are all human deep down and some are more willing to revel their humanity and it is a great thing when biographers are able to capture that for other generations of readers to enjoy.