I haven't read it, but
Defying Gravity (https://www.amazon.com/Defying-Gravity-Creative-Schwartz-Godspell/dp/1540031462) has a chapter on
Pippin -- presumably from Schwartz's perspective, which may or may not suit your purposes (the biographer seems to be, quite literally, the president of his fan club (http://www.theschwartzscene.com)). Apparently the book was updated for a second edition last fall, so it likely also includes some new material about the 2013 revival.
From the "Look inside the book" preview on Amazon:
Quote from: Defying Gravity: The Creative Career of Stephen Schwartz, p.423)"My attitude toward [Pippin] has changed over the years, probably because I am no longer twenty-four years old... I have come to appreciate the theatricality and subtextual richness of undercutting both expectation and sentimentality, which is something Bob Fosse understood better than I at the time. Indeed, I have come to joke that in many ways, I have ironically become the guardian of Bob's vision, and that somewhere he is looking up and laughing."
There's a recent interview on that fan site in which Schwartz discusses Fosse,
Pippin, and
Fosse/Verdon:http://www.theschwartzscene.com/2019/04/25/stephen-schwartz-comments-on-fosse-verdon/
And the site also includes this list of
Pippin-related resources (scroll down for "More Books on
Pippin"):
http://www.musicalschwartz.com/pippin.htm
Also, the
Pippin section of Schwartz's own website includes several Q&A pages about the show and its history (see "Resources," at lower right):
https://www.stephenschwartz.com/works/pippin/