Have a look here: http://www.amazon.de/Introduction-Bioinformatics-Arthur-Lesk/dp/0199651566/ref=sr_1_1?ie=UTF8&qid=1392720883&sr=8-1&keywords=bioinformatics
Its an apparently simple question, although very difficult to answer. You will find tons of books on basic bioinformatics. But most of them don't cover what entails as "Bioinformatics". Simply because of the very broad range of applications served by the field. My suggestion is that you refer to "Bioinformatics Basics: Applications in Biological Science and Medicine. Lukas K. Buehler and Hooman H. Rashidi".
This Book will provide you a snapshot of all aspects of bioinformatics ranging from biological databases to statistical techniques required for complex data analysis. I hope this information will be helpful to you.