This book is often recommended to biologists and clinicians undertaking clinical research. Although it does contain pretty much everything you need to know in order to design and analyse a study, it is not the sort of book that is easy to read cover to cover. The language is heavy going and old fashioned.
For complete beginners, I would have to recommend Primer Of Biostatistics by Stanton A Glantz. By using rather zany examples from the planet Mars, the reader does not get bogged down in all the medical terminology, unlike the examples in Altman. And it is a book you can sit down anywhere (on the bus or tube) and continue where you left off.
To summarise: If you are a medical student who needs to pass a test in medical statistics, go for Glantz. If you are starting some research and have no knowledge of medical statistics, go for Glantz. If your budget allows, get Altman for reference (otherwise borrow it from your library, or someone who gave up trying to read it).