Genesis, the first book of the Bible, presents the story of how God dealt with His faithful and chosen ones from the time of Adam right on to the time of Joseph and his brothers in Egypt.
The book of Exodus simply continues the story. The families of Joseph and his brothers multiply and within a few generations become so numerous that the new king of Egypt feels politically threatened and succeeds in making them slaves.
As the years slowly pass, their bondage becomes almost impossible to bear. But, finally, these slaves (the new nation of Israel) miraculously escaped from Egypt. A short time later, they gathered around the foot of Mt. Sinai, and they heard the voice of God thundering out His Ten Commands.
Moses, the author of the book of Exodus, was an eyewitness of these historic and miraculous events. It's no wonder, then, that he filled this book with so much vital truth, drama, and suspense.