@Moondeer The problem was that $cover_width is picking up the full path to the image. So, in your book partial, I replaced

<img 
      class="bookshelf-book-cover"
      src="https://micro.blog/photos/{{ $cover_width }}x/{{ .cover_url }}”``
      align="left" />

with

<img 
      class="bookshelf-book-cover"
      src="{{ .cover_url }}"
      align="left" width="{{ $cover_width }}" />