Scrum and Kanban are not rivals. If your Scrum Sprints are getting bogged down or missing the mark, combining Scrum with Kanban might be the Agile answer. This article from Kert Peterson provides a comprehensive explanation of how Kanban can improve your Scrum process with Kanban Task Boards, Team Boards, and a better focus on customer needs.
Although many might tend to limit the concept of agile requirements to “user stories”, this book by Dean Leffingwell reminds us that there could be more than just a post-it on an information radiator when we talk about Agile requirements. The title of one of the initial chapters is “The Big Picture of Agile Requirements” and this book provides it, together with the small details that can help you write better user stories.