>what is it about a book cover that attracts you to pick up a copy of a book by an unknown author?

That rarely happens. However, the Bill Gregory cover illustration of Laura Joh Rowland's Shinju did attract my attention. It is a medieval Japanese style drawing of what looks to be a parade over a bridge with a city scene in the mid ground and a snow tipped mountain in the background (although it may just be a busy bridge, upon closer inspection).

That did not lead me to buy the book, it led me to read the back cover blurb, which did lead me to buy the book by an unknown to me author.

Interestingly, there are a number of other editions of that book with different covers. If I had come across one of those other editions I would not have read the back cover blurb and would not have bought the book.