Mооnst@r
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In the paths Textures/HeroScreen2/Inventory/Cast and Textures/HeroScreen3/Inventory/Cast (I rechecked with the original data folder; the paths with this folder and the final file are actually two, I don't know why), the Normal.dds file has been modified – the resolution has been doubled (from 69x69 to 138x138). In the Normal.xdb file in both paths, this change has been taken into account.This file is responsible for the image of the spellbook icon in the hero viewing menu.
There are related files in the paths UI\HeroScreen2\Inventory\Cast\Normal.xdb and UI\HeroScreen3\Inventory\Cast\Normal.xdb, but I don't see anything interesting there that is related to the resolution and needs to be changed.
You can simply create mipmaps for the texture and then take the x69 (the desired size), or, if you really need a larger image size, as shown in the second screenshot, add Scaling to BackgroundSimpleTexture.
I've attached your mod with this fix.