Mostly day one writing advice with no specific relevance to writing in the mystery genre.
20-25% Half decent and less talked about advice, but again not specific to writing mysteries.
If I was generous I’d say about the same of specific mystery writing advice, but if you don’t know how to write a mystery before you read this book, I don’t think that’ll be any different afterwards.

Also a healthy sprinkling of “advice” like ‘write a diverse group of characters,' back to back with advice to 'only write about minorities if you’re from those cultures.’
So the advice is to… what? Aim to be born into as many minority groups as possible?