If I had to flee my home and had room to take with me only ten books, knowing that I might not be able to acquire any more Catholics books again, this would be one of them. (And I'm counting my Cornelius a Lapide Commentaries on the Gospels as five books.) It is simply that good.
How can I explain? Jesus says that we must become little children in order to enter the kingdom of Heaven. In my opinion, Therese shows and explains to us how to do that better than anyone ever before-- after Our Blessed Mother. So, if you really want to get to the heart of the Gospel, go to Therese. That's why Therese is a Doctor of the Church.
However, although the book is simple, it can be misunderstood. For example, as a teenager, I didn't understand the spirit with which Therese did all her little sacrifices and ended up going a bit cuckoo over it. I recently finished a companion to The Story of a Soul called "A Retreat with St. Therese" by Fr. Pere Liagre. I would include this in one of my top ten books as well due to how well it explains Therese's teachings. I would recommend reading read Story of a Soul first and then following up with Fr. Liagre.