In addition to the above answer, Winston has wanted to find a conspiracy or group working against the government for a long time. He thinks he has found this in O'Brien. Remember, Goldstein, the author of this special book, is alleged to be such a leader of conspirators.
Winston's average ability makes him ill-equipped to deal with conspiring against the government, but his ability to still think makes us as a reading audience wonder at his ability to achieve something great. Not knowing what he will do with the item, for Winston this book is a symbol of possible freedom or release from this world. It is a potential key to working for a cause to thwart the wrong of Big Brother.
In the previous chapter, O'Brien has told Winston that a man will give something to Winston by pretending that Winston has dropped his briefcase. The man will be giving Winston a copy of Goldstein's book.
Goldstein's book is of special importance to Winston because Winston wants to understand the way his society truly works. The book lays out the whole thing -- it tells what the Party's goals are and it tells (in great detail) how the Party goes about achieving these goals. So, by reading the book, Winston is going to be able to completely understand his society and (he hopes) how to change it.
In this chapter of 1984, the item in the briefcase is a copy of the book The Theory and Practice of Oligarchical Collectivism by Emmanuel Goldstein. According to O'Brien in the previous chapter, reading this book will make Winston a full and complete member of the Brotherhood, the underground resistance movement that seeks to bring down the Party. So, for Winston, this book is very significant because it will enable him to join the fight against Big Brother and (finally) give meaning to his rebellious thoughts.
To enhance this sense of significance, Winston has the book in his possession for almost a week before he has the opportunity to take it out and read it. This intensifies his anticipation, as is shown by the "tingling" he experiences every time the briefcase touches his leg. This develops the idea that the briefcase is truly special to Winston and is an item worth waiting for.