With regard to the Sunday Times bestseller of a book, I’ve done some research.
1. The list is compiled based on weekly sales and you only need to top the sales rankings for your particular genre ONCE to be able to say your book is a Sunday Times Bestseller. Pre-orders also count so a book that has around 10,000 pre-orders will likely become a Sunday Times bestseller immediately upon its release, hence the push on pre-orders.
2. The author does not receive very much money per book sold. There are various figures online if you look into it; obviously it’s impossible to know exactly how much Rachel earns per book sold but the general consensus is that it’s not very much. I’ve seen a couple of sources state that it is around 25p per book.
3. The first thumbnail is an estimation of actual sales based on the Amazon ranking of Rachel’s book (I found the ranking and entered it here to get this result). It is estimated that the book sells 143 copies a day, and it is currently priced at £3, which totals £429 a day. If the author is indeed receiving 25p per copy sold, this equates to around £35.75 profit per day, before tax. This figure will fluctuate as much as sales do and will definitely decrease as the book gets older and more second hand copies come into existence.
So, I think it’s pretty safe to say she’s not making a fortune from her book, regardless of it being a “bestseller”.
More info here:
https://www.google.co.uk/amp/s/www.entrepreneur.com/amphtml/280520