Xous Operating System

Here's their 39c3 talk about Xous: https://media.ccc.de/v/39c3-xous-a-pure-rust-rethink-of-the-...

Interesting, didn't hear from this system so far. Seems to be funded by the EU. Apparently it is written in pure Rust since 2020, and Andrew "bunnie" Huang seems to be involved.

Is there a PDF version of the book (https://betrusted.io/xous-book/)?

What problem is this solving? Are there no OSes for medium embedded systems? Are they too expensive?