Now a book lives as long as it's unfathomed. Once it is fathomed it dies at once...once it is known, and its meaning fixed or established it is dead. A book only lives while it has power to move us, and move us differently; so long as we find it different every time we read it. Owing to the flood of shallow books which really are exhausted in one reading, the modern mind tends to think every book is the same, finished in one reading. The real joy of a book lies in reading it over and over again, and always finding it different, coming upon another meaning, another level of meaning...It is far, far better to read one book six times, at intervals, than to read six several books. Because if a certain book can call you to read it six times, it will be a deeper and deeper experience each time, and will enrich the whole soul, emotional and mental. Whereas six books read once only are merely an accumulation of superficial interest, the burdensome accumulation of modern days, quantity without real value.
Note here that it's required that you find it different when rereading a book. Or you might say, it finds you different. I have a few favorites that I return to occasionally but only for comfort and when I read them I am the same reader and it is the same book fulfilling an emotional need. But it is not revealing a deeper sounding...rather, allowing me to stay at one level, at my ease, as a kind of pacification (something akin to eating for comfort).
My first experience like this came with Conrad's Heart of Darkness. It was intentionally opaque and perhaps that's cheating, but while its action in the jungle and on the river remained the same, the lines, the words, the telling of it, would slip away in different directions and then sneak back in bearing a cloak of allusion.
I think that writers when deeply read, in any field, use all their experiences as readers when they write. You know the saying "you are what you eat", well this seems applicable to books as well. They are the culmination of the writer's reading history, what he/she has eaten. This doesn't mean breadth of reading...it means reading deeply, repeatedly, being in thrall to the words and meanings and finding them internalized within you and then filtered through you back out into the world through your mouth or pen.
What books keep you sounding, diving and surfacing with fresh depths in your maw?