Learning Medicine by Peter Richards and Simon Stockhill was one of the many books I used to help me decide whether I wanted to becoming a doctor.
It is informative yet concise enough to be interesting and covers many issues and questions that students like me might have, including why medicine and why not, requirements for entry, choosing a medical school, interviews, what medical school is like and what happens after medical school.
I think this book is very clear and accurate. It's a must-read book for those who are thinking of going into medicine. Make sure you get the newest edition though, otherwise the information will not be up-to-date. I think the newest edition is the seventeeth edition (as of November 2011).
The book goes into detail about medical school is like and helps with answers to interview questions and writing your UCAS personal statement so it is an essential read. The book also talks about life as a doctor, and the different stages of the medical career (i.e. becoming and F1 and F2 doctor and how to specialise) so it also applies to people who are currently training as a doctor.
What is most important for me, though, is that it has made sure that medicine is the right career for me, as it goes through the essential qualities of the doctor, like working well in a team, empathy, etc. Furthermore, its use of case studies are integral in the book and it shows what medicine is like from people of different backgrounds, and there were many case studies whose stories related to me, which really helped me to decide whether medicine is for me.