It may sound trivial but it’s astonishing how many students don’t do it.
Here I am talking about literature in the home language (German for Germany, English in England) as well as foreign languages (English and French when I was at school).
You may feel like you know the story well enough because you’ve been discussing it in class for the past month. You can even participate in class without the teacher knowing that you have only read the first three chapters. However, it will not work in the exam.
1. In the exam, the teacher can tell if you recognise the details of the story, the exact reasoning a certain character gave, etc, etc. You are deducted marks for what would have been fine in class.
2. Not just Wikipedia but any other source giving you a summary is not accurate. This includes your classmates!
3. Many classics which are used at school have an additional book published including a complete summary, interpretation and other various topics which are often discussed. This can be useful when preparing for lessons but I have seen all too many people reading the complete interpretation but not the story!
I myself never read these interpretations. Instead, before an exam I read the complete book again, making sure I knew every last detail.
4. Once you read a book properly, you will realise that most of what seems like complicated analysis is common sense. By knowing the details of the story, you save time and effort on memorising the topics which are discussed.
I may be repeating myself but all additional sources cannot replace the original book. Think back a little. Have you been reading summaries? Or god knows how many interpretations? Then cross all those out of your program and read the book again, properly! I mean it!