OneNote Service API
dev.onenote.com
dev.onenote.com
To address the "one way" API limitation, I would expect Microsoft to add more endpoints over time and I think it's not a bad strategy to start with that one. Adding a way to post notes to OneNote already enabled a lot of cool integrations...
Having external tools, both hardware and software, is a huge part of building a memory-augmenting product and I'm really happy that Microsoft is getting it right - OneNote has always been a really interesting product, but it suffered from weird positioning and always felt like an add-on to Office more than anything else. Now that it's being marketed as more of a standalone (and free!) product, I think it has a great chance of becoming much more mainstream.
OneNote itself was launched over ten years ago as part of the Office suite, while the OneNote service is just launching and has been developed completely internally.
You can also DM me @jmslau if you like.