@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 }}" />