Can anybody recommend a source of information to evaluate the DevOps maturity of an organization?

DORA is pretty canonical depending on your definitions.

We like to think in terms of capability rather than maturity. What this means is you use the DORA research as part of your continuous improvement process - choosing to look at capabilities to solve the challenges you face right now (instead of adopting all of them in a specific order). The DORA Core model gives you ways to measure your performance and plenty of capabilities you can choose from to get your next improvement (usually through an associated j-curve as we all need some practice with new things before they start paying off). https://dora.dev/research

If you’re looking at PE as well, then this would be a nice addition on top of DORA --> https://tag-app-delivery.cncf.io/whitepapers/platform-eng-maturity-model/