It's always astounding that we find that the old ways of doing things are far superior to the new... Somewhere along the line, we lost the knowledge, and it's only by luck that we rediscover how it was done.
Case in point - Roman cement. Two thousand years, and it's still strong - literally. Compare it to today's versions which last a few decades at best.
Not that older is necessarily better, but it does indicate how knowledge can easily be lost.
And why it's important that you work to preserve it.
I tend to say half-jokingly that I'm accumulating a library so that, PSHTF, I can hand a couple of books to someone, say "Learn this". At least then I'm not relying on luck to have those with the right skills just turn up - as in quite a bit of bad post-apocalypse fiction. Of course, it does mean finding worthwhile books - a wide range, and reasonable depth. Not a difficult task for a bibliophile, except to the wallet.
There's also the problem of working out what is necessary to have books on - and trying to find books on the subject. Have you any idea on how hard it is to find any references on building cotton gins? Here I am, growing plenty of cotton, and no time to manually pick every seed out of the fibre. Spinning wheels are easy enough, and even looms can be gotten as kits, but a good gin in hard to find.