Do not have Mary's book. Was initially interested but at the time reviews also held me back.
Have Scott's. It is a VERY quick read. There are many instances where a page is only half filled, lots of space filled with lyrics, other space filled with photos.
There are definitely some parts of the book that have good detail, and I personally learned many things I hadn't known (or hadn't known enough about at the time). But there are also many topics that just get glanced over.
My final feeling of the book was just that I had wished there was so much more. Reading the ghotswriter's story about how the book was worked on, I now understand why it ended up that way. Difference is, you have this knowledge going into it so perhaps your expectations are better tailored.