This is the best introduction to lucid dreaming that I have come across.
A dream becomes lucid if you realise you are dreaming during the dream.
The book is readable, only 77 pages long, and comes with a CD which provides a second line of instruction.
Stephen LaBerge is a world authority on the scientific study of lucid dreams, with deep personal experience and an excellent teacher. He is an enthusiast for remembering dreams, especially lucid dreams, and his enthusiasm is catching. He gives ways of recognising when you are dreaming, during the dream, and discusses some of the uses of dreams, especially lucid dreams.
Of the four books on lucid dreaming that I have read, my advice is to start with this one. The other three books are: Lucid Dreaming, by Paul and Charla Devereux; Exploring the World of Lucid Dreaming, by Stephen LaBerge and Howard Rheingold; and The Tibetan Yogas of Dream and Sleep, by Tenzin Wangyal Rinpoche.
I strongly recommend this book.