So what does this one depend on? First it depends on whether they really are impressed with your work or if they are allowing their name to be used out of friendship. I’m more impressed when the pubbed writer makes the initial contact bragging on the friend and asking if I would look at their work. I always do.
But sometimes when the actual submission arrives and I see the work of the submission can’t hold a candle to the work of the pubbed author and I know it is simply a “friendship endorsement.” Other times the submission lives up to the endorsement and that encouraging word by the published author certainly carries a lot of weight with me. Even more significance is when a known author or celebrity has agreed to write a foreword or introduction to the book. When they agree to do this they are in essence putting their own reputation on the line being published in the book with the writer. That has to carry weight.
It’s another case where networking is so important.