If there is a book coming out from one of your writerly friends via traditional publishing that you plan to purchase anyway, the best way to support them is to preorder. Publishers look at preorder numbers to determine their print run. Early sales are also important. The first two weeks of a book's shelf life are the most important. This article by Sophie Perinot at From The Write Angle explains why.
I did not know this. I'm inherently guilty of procrastination, but I did not know that my preorder could make a difference. For all the times I said to myself...I like the sounds of this and I'm going to buy it....later. I'M SORRY!!! PREORDER is the word of the day.
Thank you Sophie for opening my eyes.