@jamco.eth
We (finally) opened up the contracts repo for Onit. Many PMs work as order books but Onit is an AMM, and it implements a market mechanism that hadn't been tried before.
The release here is a beta (not deployed yet and will change before then) for v0.0.4. Might work on getting tags for the previous 3 releases published too when I get a chance. Open to feedback before then so please have a look!
You can build on the contracts directly, or if you want an easy way to integrate them into your app, we have an API and SDK.
https://github.com/onit-labs/pm-contracts