I'll keep this brief:
My first foray into the Dorothy Sayers biography project was not a great one.
Within only a few pages of the Introduction, the author got a major aspect about Sayers' most famous work wrong when he described a the character Harriet Vane as the lover of Lord Peter Wimsey, which is not just wrong but it's a mistake that easily shows me that the author may not have understood the books. It's a long story, ... but it is a detail that either is a genuine mistake or a clear indication that the rest of the biography was not going to be good.
I'm not sure, but I don't believe the biography was a great one: for one, a lot of the book seemed to rely heavily on Barbara Reynolds' biography, and I am sure there was at least one memorable passage where author used a description - almost word for word, and not as part of a quote - from another biography by James Brabazon. The description was memorable because it was quite unflattering and it made me wonder how the author could possibly pass such judgement on Sayers without having known her in person.
Also, the author professed to have carried out extensive research in the archives that hold many of Sayers' original letters, etc. but for all that research, I didn't think there was that much original thought or new evaluation of Sayers' material, and the book felt quite flat and facile.
If looking for a biography of Sayers, head straight to Barbara Reynolds book. It has much more to offer and was written by someone who not only knew Sayers in person but also had access to a lot more of the letters and other material.