Thomas D. Cook & David L. DeMets 
Introduction to Statistical Methods for Clinical Trials [PDF ebook] 

This text provides a solid understanding of the various statistical topics relevant to the design, monitoring, and analysis of a clinical trial. It covers basic experimental design, sample size, randomization procedures, interim and survival analyses, quality of life assessment, and longitudinal methods. The book describes all phases of a clinical trial and explores the advantages of and objections to the intent-to-treat principle as well as its implementation. With exercises at the end of most chapters, it also illustrates methods using examples with real data from a variety of disease areas, including heart and lung diseases, cancer, and AIDS.