I don't know if Booth has done any new interviews with Warnock for this book - or if it is all based on stuff already public.
I doubt it will have anything fresh or interesting to say about the Sala tragedy.
However, in a few years time it is likely that Warnock will update and extend 'The Gaffer' to cover his managerial career since 2013. If he does that (with his ghost writer) he may well open up.