It's important to note that:
* Einstein was a voracious reader with wide-ranging interests: He enjoyed philosophy, literature, and science, and likely read countless books throughout his life.
* Personal preferences are often private: It's possible Elsa knew what his favorite book was but never shared it publicly.
* There are no reliable sources: While some online resources might claim a specific book was his favorite, there is no credible evidence to support these claims.
Therefore, it's impossible to definitively say what Albert Einstein's favorite book was.