For manga, I’ve found Mihon to be nicest, by far, and it supports the API. For books, I am currently “stuck” on koreader on Android (which “only” supports OPDS-PS). I do most of my reading on a reMarkable currently, and that has no supporting client. Writing one is on my to-do list, but it’s a bit daunting of a task…

Here is a pretty good list of what is supported where.