Books are like friends to me. Some are inspiring; some are amusing; some are uplifting; and etc. There are times when I am in need of a certain "friend" and I'll reread a book all over again. These books never fail to satisfy me provide whatever entertainment for which I am looking.
If by spirit you mean "meaning" then I think you need to read with an open mind and an open heart. Good literature unfolds a captivating plot and an interesting cast of characters so that the events that play out reveal something about human nature. If we stand back from judgement and really just try to be in the characters' minds and lives, then I think we can learn from their experiences. That is what I mean by 'open heart.' As for 'open mind,' I am referring to an intelligent attention to the conventions of literature, such as setting, symbolism, figurative language, structure etc. When reading is a healthy blend of the two, then meaning becomes all the more clear.
I'm not sure what exactly you mean by a "final" way to understand the spirit of literature but I'll offer to following possibility...
It has been proposed that one of the central purposes of literature is to explore the human condition In essence, then, literature seeks to explain and offer examples of what it means to be human.
The only way that I think we can access the sense of feeling is by experiencing it first hand. When we are children, must of us enjoy reading about ourselves or others just like us. We feel a connection between the words we see on the page and our own history. However, the true "spirit of literature" in my mind is when we experience something in our lives that we feel we've read before. When some event happens that we feel we've already explored and experiences vicariously because of something we've read. In my own life, that has happened on several occasions. And each time, I am struck by how wise and enlightened I've become because of the literature that I have read.
So, then, the only way to understand the spirit of literature is to read widely with the idea that what we read may help to shape our understandings of future events in our own lives. Of course, we have no way of predicting what our future holds, but as we live our lives the connections between what we read and what we experience will come to enlighten us.
I agree that the reader must find the purpose of the piece of literature and how it achieves that purpose. I'm sure that perceiving the spirit of a particular work is different for everyone. For me, finding the spirit of a work of literature is about finding a connection. I definitely don't think that you can do this with every book you encounter. There are only a handful of books/writers that have made me feel this connection.
I'm not sure there is a final way to understand the spirit of something. By its nature, the spirit of something in rather intangible and indefinable and inexplicable. In other words, the final understanding of the spirit of literature is how it makes you feel, how it makes you think, and how it changes or reflects your perspective on life.
The spirit of any literature? The spirit of all literature? One particular book?
I think the key to understanding the heart of a piece of literature is to figure out the author's purpose. Who is he? Who is his target audience? What is his foundational message? How does he get this message across?