The paper Case Study of Customer Input For a Successful Product was published in 2005. The author Lynn Miller depicts a real case she experienced in Alias which they combined Agile and interaction design together to come out a successful product. Several commonly used customer input methods are elaborated in this paper, really worth reading!
I got the name of this paper from a video presented by Johanna Kollmann on GOTO Copenhagen 2012. Johanna herself is an experienced UX designer and she recommended this paper on one slide during her presentation.