The Bible. Even if you don't care for it's spiritual implications, there is a rich history that is used by archaeologists, and many face-value lessons of cause and effect, and even recommendations on how to manage a business and employees. The beautiful non-spiritual message is that humans don't change, but we can still learn from histories mistakes.
I feel like I've got to read it eventually just so I can be up to speed on all the millions of references to it in other works of art. In a college class recently, we read beowulf, and I was confused what was meant when grendel was described as being from "cain's clan" - I was raised non-religiously by parents who had both been burned pretty bad by institutional religion, so it was pretty much only me and an asian immigrant in that class who needed the reference explained. (Granted, I was one of the few non english majors in that class, which also probably affected my lack of understanding.)
The main cultural touchstones are disproportionately concentrated in a few books too, you can get a lot of them with just a dozen or two hours of reading.
Genesis specifically is packed with common references, can be read in a few hours, and is fairly engaging and accessible as a coherent piece of literature in its own right.
Ecclesiastes is like five pages and possibly the most quoted thing across european cultures. So many literary references and even common idioms come from there.
After that any one synoptic gospel + john + acts will set you up to catch a lot of christianity-specific cultural references. And then revelation imagery comes up a ton in pop culture, music, film & tv.
All of what I mentioned is about the length of a medium-short novel and would set you up to catch probably the majority of allusions to the bible. You'd be missing some major stuff like moses, david & solomon, plus a bunch of misc but influential stories like jonah & the whale, samson etc. But for bang for your buck it would get you pretty far.
Going through life without having read through at least some of the KJV Bible and Shakespeare would be like watching The Wizard of Oz in black and white – sure, you'd be able to follow the plot, but you'd be missing out on the color experience. Similarly, without having some foundation in those two foundational works of English language, you'd be missing idiom, metaphor, and allegory that you might not even realize is there. Imagine trying to read President Lincoln's speeches without getting his references to scripture. Imagine trying to read Faulkner's Sound and the Fury without getting the titular reference to Shakespeare and the structural reference to the Gospels (the same story told by four different authors).
If one is looking for a much more readable Bible, versus those low quality, thin paper, small typed copies, I'd recommend checking out Bibliotheca. Also has one of the best videos I've seen done for a Kickstarter campaign.
The single most significant fact supported by the Bible is the life, death, and resurrection of Jesus Christ. History agrees that Jesus was a real man. History agrees that He was significant in the region. The eye witness testimonies support (but don't prove) the resurrection, but consider this - many men and women were executed in the first century because they would not recant that they were eye witnesses to Jesus' crucifixion and resurrection. If it were a lie, why would so many take that to the point of their own death?
> If it were a lie, why would so many take that to the point of their own death?
Consider a weaker question: if it weren't a lie, why would so many take that to the point of their own death? While most people are usually opposed to lying, if they felt their reason for lying was important enough - and if they're willing to die for it then it must be - they could easily overcome that.
I would argue that the average person would lie to preserve their life. Not all, of course, that's a pretty wide stroke to paint many people with.
The resurrection of Jesus Christ is a dividing point to all humanity, and there are many willing to die on either side of the fence. While it has been contested many times, there is no definitive proof that Jesus was not resurrected, and there is pretty solid evidence that Jesus did die on the cross (as opposed to old age or some other natural cause). We also know that the Jewish leaders at the time of Jesus were furious at Christianity, and had a serious campaign to destroy and discredit it, yet it survived. You'd think that with their financial and people resources, they could have recovered a body or exposed Jesus as a fraud.
