Artemisia, the Greenspan book "The Child With Special Needs" has long been considered the "Floortime Bible," but his newer book "Engaging Autism" is much more detailed and offers a scientific study demonstrating the effectiveness of Floortime. If the Floortime concept of "circles of communication" seems vague, then I highly recommend the book "Giggle Time" by Susan Sonders. She gives page after page of solid examples of verbal & non-verbal communicative turn-taking. She proves beyond any doubt that all behavior is communication.
Some parents already practice Floortime intuitively and don't need any instruction. It's all about going back to our instinctive playfulness.