Marc also wrote the first biography about Terry (The Magic of Terry Pratchett), a few years before Rob's biography came out. They are both great books, and Marc and Rob are now pretty good friends (I've interviewed/seen interviews with them both).
Marc also turned his biography into a comedy lecture show (also titled "The Magic of Terry Pratchett") which has done extensive touring of the UK over the past several years, with a few international runs including at the Adelaide Fringe in 2025, which is where I managed to see it (and Marc came out to dinner with our local fan club after the show too).
I'm quite excited about these new books - part 1 comes out this year, and part 2 next year, because Marc is actually doing a deep dive into /all/ of Terry's books (not just Discworld), so it's less of a biography and more of a critical analysis bibliography - and given that Marc is not just a comedian and author, but a musician and a newspaper columnist (you'll often see his name on articles relating to Terry), I'm sure his insights will be very good to read - even if you don't agree with every one of his opinions.
If you want to know what the contents of this book will be like, Marc has put a sample chapter (about Strata) up on his website here:
https://www.marcburrows.co.uk/terry-pratchett-readers-guide
He also recently published an extract of his "Equal Rites" chapter on various social media platforms.