Now I am not sure about uncountable infinity in Umineko but the city of books I think, is quite comparable to the wanderers library. It contains characters that can make stuff like catboxes. Catboxes can be infinite with infinite universes/fragments and Bernkastel's avatar managed to make one that surpassed Beatrice's catbox which is infinite/endless. She did it all in a single fragment while seeing it as fiction. Battler has endless nine which is allows him to infinitely evolve through infinite layers. The city of books contains characters like him in a single line. Tl;dr, the city of books is a place that is immensely above infinite layers of existence and it contains countless other catboxes and entities.