There's never any independent evidence, and to the extent that you can test the claims, they don't hold up. For example, if you look in the parking lots of these buildings at night or look for which office windows have lights on, it's obvious that they're nearly all empty, and that's always been the case. But both sides in the argument ("yeah I worked that hard" vs. "no one should work that hard") have an incentive to agree on inflated numbers, so they can argue about the rest.
I do remember one startup where the CEO would have a tantrum if someone left before midnight, but that was only for a brief period (the situation was unstable), and even at the peak of that insanity, no one worked close to 80-90 hours a week. People would just come in late the next day. They certainly thought they were working 80 hours though.
Having occasionally seen people working that much outside of crunch - only at one startup and in the game industry, admittedly - I've just been taking those claims at face value.
If you're working 80-90 hour weeks, which isn't a crazy number from what I can tell at a lot of these startups, you aren't reading two or three books a week.
Or rather, it's very unlikely you have the mental capacity left to process what you're reading unless its mindless pap - which is fair, I can dig mindless pap - but that is almost the same creature as watching Netflix or 3 am infomercials.
If you are working two full time-jobs worth of hours each week and are still managing to read books, good job?