You might associate Albert Einstein first with his theory of relativity, but the frazzle-haired genius was also a voracious reader, being particularly fond of works by Johann von Goethe. Out of all the titles that could have caught his eye, though, just one earned a place on Einstein's nightstand--Don Quixote, by Miguel de Cervantes.
If you've never read the story, beware, I'm about to give some spoilers. The title character, Don Quixote of Spain, becomes enraptured with the ideals of chivalry he reads about in books, and endeavors to keep these ideals alive by becoming a knight. He becomes obsessed with defending the honor of a peasant woman, Dulcinea del Toboso, and embarks on multiple journeys with his squire, Sancho Panza. Along the way, just about everyone thinks he's crazy, but Don Quixote doesn't give up his beliefs until the very end of his life.
Cervantes remarked that he wrote Don Quixote as a parody of the books of romantic chivalry being written at the time, volumes he considered to be "vain and empty." The bigger point was an acknowledgement that Spain's society was, in fact, dramatically shifting away from older concepts. Nevertheless, the story has multiple points that likely endeared it to Einstein.
A defense of morals.
Don Quixote embarks on his quests in part because he's bored and lonely, but also because he believes--perhaps because of his madness, or in spite of it--that the world is in real need of a defender of truth. Einstein too spent considerable time contemplating the junctures of science, philosophy, and justness, especially as the Nazis rose to power. In fact, one of his other favorite books was Benedict de Spinoza's Ethics, which seeks to unpack the meaning of moral life. And as an entrepreneur, like Don Quixote, you should know what you stand for and stay steadfast in your ideals to gain the trust and loyalty of those around you.
Persistence in the face of doubters.
Others mistreat Don Quixote on his quests because they think he's lost his mind. But he doesn't let them break his spirit. He continues to work toward his goals and fight "battles" he sees as being filled with valor. Einstein likewise had to work against critics, such as Arthur Patschke--these critics found fault not only with Einstein's theories, but with Einstein himself as a Jewish person. As an innovator, you'll have persist even if others initially mock what you're doing.
Being willing to tackle big problems.
Even though Don Quixote has multiple battles, the most famous is his bout with a field of windmills. He charges at the towering structures bravely despite their Goliath status, convincing himself they are actually bewitched giants. Similarly, Einstein's theories were no small feat to develop, taking years to work out (the theory of relativity took a full decade). Good entrepreneurs also don't shy away from big ideas just because they are time consuming, complex, or resource-intensive. They work little by little, step by step, to figure out processes and solutions.
Don Quixote doesn't go through his quests alone. He enlists the help of Sancho Panza through everything, finding high value in his service and friendship. Einstein relied on others too, such as Michele Angelo Besso, Neils Bohr, and Max Born. Great leaders also do best when they find mentors and mentees.
Don Quixote is meant to be lighthearted. But there are serious themes in it that are highly inspirational. At the same time, it is, as Einstein is said to have described it, a "drama of ideas." The satiric clash of ideologies, together with a hero who demonstrates so many laudable traits in the face of lunacy, made it perfect, relaxing amusement for Einstein's genius mind. While the classic text certainly will require your commitment (it's a 1,000+ page behemoth), it can be the same to you. When in doubt, remember this quote, likely attributable to Miguel de Unamuno, a writer who discussed Don Quixote in his work: "Only those who attempt the absurd will achieve the impossible."