Hey.

General knowledge includes both  Static GK as well as current affairs. For  a good grasp in static GK the most important fact is that you must use a very reliable and relevant source. The best books for static GK are Pearson's general knowledge book. It comes in a consized size too, so you can use either according to your requirement. Lucent's general knowledge book is also very usefull. For an interesting way to retain it you can play online quizzes or download apps for quizes on GK.

For current affairs, first of all develop a habit to read newspapers daily. The Hindu, The Economic times etc are considered to be the best newspapers for this purpose. It not only builds a strong base for General knowledge but will also help you in your vocabulary. You can also team this up with going through monthly current affair magazines to brush up what you have read.

Other than this, if you preparing for any specific entrance exam, then the level of preparation and the source for it may differ accordingly.

Best of luck