1) The Analects of Confucius. The Analects, along with the rest of the Four Books and Five Classics, make up the core of Confucian ideology, which formed the basis of East Asian government and philosophy for more than 2,000 years, and still serves as a major influence on East Asian thought. Pretty significant considering the important place China holds in world history.
2) Plato's The Republic. The first great work of western philosophy, and the predecessor to the works of Plato's student and ideological opposite, Aristotle. While Socrates might deserve credit for the beginning of this philosophical movement, he didn't write anything down so credit defaults to Plato. The significance of The Republic itself is closely challenged by any one book Aristotle wrote.
3) The Principia (aka, the "Mechanical Principles of Natural Philosophy), by Isaac Newton. The book that for the first time fully married advanced mathematics with scientific principles. Easily the most important scientific book ever written, and more or less founded modern science. While Einstein's three papers laid the groundwork for modern physics, they would have been impossible with without the revolutionary groundwork laid by Newton.
As for how to draw upon these books for D&D inspiration:
1) The Analects were the basis of the exam system in ancient China. If you want to have a setting where prospective government officials (or mages) need to study ancient texts to advance, they serve as some inspiration.
2) Plato's World of Ideal Forms is one of the great untapped sources of inspiration for magic systems and cosmologies. Seriously, how come it is never brought up?
3) Isaac Newton was a magician and alchemist. There has got to be something interesting in there. If nothing else, it is always a good thing to consider when you want to make magic and science opposing forces in a campaign setting.
BTW, why the heck would anyone consider Shakespeare's plays to count as religious texts? I considered listing his works, but I figured it would be too ethnocentric of me. His impact on the English language can't be understated, but his world-wide influence isn't quite as significant.
I really feel like I should list a major Indian work, but I don't know them as well as I should. Most of the big ones do count as major religious texts. However, I will give an honorary fourth pick to the Ramayana for being one of the most awesome and inspiring mythological stories I have ever read. If you want to know what Epic level D&D should look like, read the Ramayana.