The field of Customer Success is fairly young. This is a role within software companies which tasks folks with managing long-term relationships between their employer, the vendor, and a set of customers. Folks working in this role will benefit from an understanding of most aspects of their employer's software, the details of their customers' businesses, etc. I'd tend to say that because you'll likely be working across teams you'd want to have a foundational understanding of "devops" knowledge, see my devops reading list for ideas there. Depending on the product you may want to have some domain expertise in other areas - some programming experience working with (against) REST APIs will probably be helpful, for example.
Customer Success is a foundational text in this practice.