As Eric pointed out, look at the suggested reading materials. I'm earned my PSM1 and SPS and both times, the suggested reading material was more than enough to learn the content and pass the exam. Also, don't just read the material once, especially for an advanced exam. You should have the Scrum Guide nearly memorized and have a strong understanding of the content of the other reading material.
The advanced assessments are based on knowledge from the sources listed in this topic, as well as real-world knowledge of working in the Product Owner role. I strongly recommend that you attend the in-person PSPO and PSPO Advanced courses where you will learn from knowledgeable instructors and from your peers.