Been using JuMP as a central part of a big project at work for the past 18 months and it's been great. Very intuitive to use, and lots of helpful responses on GitHub or discourse wherever there's been questions about the best way to do something. Congratulations on the latest milestone!
Really nice to see so many little useability improvements -- like easy temporary envs, syntax highlighting in dependency conflict errors, and more partially-applied functions -- as well as more significant language development on threading, stack allocations, and reducing latency.
If they might enjoy something on computing, I'd recommend
"The Pattern On The Stone: The Simple Ideas That Make Computers Work" by W.Daniel Hillis. It's very clear and well written, is quite short but covers a lot and can be enjoyed cover to cover more like a novel than a textbook.
Automatic differentiation. It's useful to so much computational work, but most people only get a cursory introduction to the topic (a rough intro to the minimum they need to know), whereas really understanding it seems to open up a lot of research.
For being immensely practical, I'm a big fan of
1. "An Elegant Puzzle: Systems of Engineering Management"
2. "High Output Management"
3. "Peopleware: Productive Projects and Teams"