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I found Openreads, an open source app for keeping track of book collections and reading progress, all locally. I have rarely seen it mentioned here, but I think it's pretty great for its purpose. I miss the ability to add custom fields such as series info, but it's a planned feature. The latest release was 4 months ago, but development will be picked up again soon according to the developer. (4 months is not a very long time anyway).
If anyone know of any alternatives (preferably also open source) to Openreads, would be happy to hear about it.