Suggested Reading for Professional Scrum Product Owner™ I

Prove Your Knowledge of Product Ownership

PSPO I includes questions from the following Focus Areas as defined in the Professional Scrum Competencies.

Understanding and Applying the Scrum Framework:
- Empiricism, Scrum Team, Events, Artifacts, Done
Developing People and Teams:
- Self-Managing Teams
Managing Products with Agility:
- Forecasting & Release Planning, Product Vision, Product Value, Product Backlog Management, Business Strategy, Stakeholders & Customers

Be very familiar with the Scrum Guide.
Your primary study resource should be the Product Owner Learning Path
Since the Product Owner works very closely with the Scrum Master and the Developers, it will be beneficial to become familiar with material in the Scrum Master Learning Path
Review the valuable series of articles, blogs, videos and more that pertain to the 2020 version of the Scrum Guide released on November 18, 2020.
In addition, you may find the The Professional Product Owner by Don McGreal and Ralph Jocham helpful.