I'm not shy about pinching other people's good ideas. Whenever I am stuck or uninspired there is one book that I always reach for first; Architecture In The Garden by James van Sweden.
van Sweden is one of the co-founders (along with Wolfgang Oehme) of Oehme van Sweden & Associates. Together they revolutionized landscape design in the United States and went on to develop a style sometimes called the New American Garden. It rejected vast, manicured lawns in favor of textured perennials and waving grasses.
This book focuses on the various man-made and natural features that give definition to a garden. With lots of photographs and an easy manner van Sweden walks us through 12 remarkable properties. He explains all aspects of the design process including how various stones were chosen, how walls and fences are used to create spaces and most importantly how all of this relates to the house.
Although van Sweden died in 2013 his firm lives on. He wrote several books on the subject of landscape design including Bold Romantic Gardens (with Wolfgang Oehme) and The Artful Garden. Highly reccomended.