hugo-book/layouts/partials/docs/brand.html

<h2 class="book-brand">
  <a href="{{ .Site.BaseURL | relLangURL }}">
    {{- with .Site.Params.BookLogo -}}
    <img src="{{ . | relURL }}" alt="Logo" />
    {{- end -}}
    <span>{{ .Site.Title }}</span>
  </a>
</h2>