Unix Programming Environment --- Good book! Yepp. Filters!
Zettelkasten! Yepp. I should have learned this while still in academia. Would have saved me an hour or two rereading stuff :-)
Learning for the sake of learning --- Well. I keep saying: "If no one ever had learned something seemingly useless, we would probably still gather in caves and freeze." So like always, there is a balance to strike. If "learning something new" is creative avoidance in place of something that needs to be done, then it's most probably a bad idea.
It's going great...my little "kasten" is growing. It was fun hiking with my german girlfriend Saturday and telling her about this. She said they have a real Zettelkasten at their office...a great big filing cabinet.
I'm like..."you are on the cutting edge, Dearest!" :D