Or, assuming all practical needs are met, which books would you bring with you if you were alone on a desert island?
This question may sound incredibly boring and overdone but I am convinced that it is worth mulling this over and really trying to answer it well. As there is a functionally infinite amount to read the need to prioritize books is apparent. Most books that are worth reading are only worth breezing through at 2x speed. Textbooks are best for serious study of a particular topic. But there seems to be a gap of the books that are so piercingly insightful and rich with wisdom that one can continue to work on them for a lifetime.
I ask as a reaction to "How to Read a Book" by Mortimer Adler. He gives this question as part of his explanation for why the "Great Books" are great. I interpret him to mean that the truly great books are worth re-reading over the course of one's life. Although this suggests that "King Lear" is genuinely more entertaining on the 10th read, what I am more interested in is which books can continue to facilitate self-actualization and lead a person to greater attunement to the nature of reality. Many books do this but can often be read well once and fully absorbed.
The minimal criterion for an answer is that you have read the book you recommend at least twice. Don't be afraid to recommend the obvious answers like "The Bible" or "The Complete Works of Shakespeare."