The author of each Gospel had a reason why he wrote and recorded what he did. In the case of the Gospel of John, the author states that he has recorded a selected number of “signs”, from “the innumerable signs” that Jesus did “in the presence of His disciples,” to convince his readers that:
Jesus is the Christ (Messiah), the Son of God, and that by believing you may have life in his name. (John 20:31)
This was why John wrote his Gospel and this was the goal he hoped to achieve, but why these particular signs and how do they show that Jesus is the Messiah and the Son of God? These are the two main issues dealt with in this excellent book.