Dear HN,
I've recently had a difficult period at work and I started observing signs of a looming burnout. I'm going to take a couple of weeks off before that happens so I can clear my mind. During this sabbatical I want to go through an interesting course or a book just for the pleasure of programming. The more impractical the topic is for real-life scenarios, the better. Just want to work on something interesting for a few hours a day while resting. Any recommendations?
Learning comes at a cost (time + sweat), and might not necessarily help clear your mind, particularly of whatever's happening at work that's clogging it up!
It's not what you asked about, but, for myself, if I wanted to clear my mind / avoid burnout, it might be worth rekindling old friendships, or finding out what's going on in local communities, or spending time in nature, or volunteering.
That said.. probably one of the most interesting things I'd suggest learning about are functional optics :)