{{- $ref := "" }}
{{- $target := "" -}}
{{- with .Get "href" -}}
  {{- $ref = . -}}
  {{- $target = "_blank" -}}
{{- end -}}
{{- with .Get "relref" -}}
  {{- $ref = relref $ . -}}
{{- end -}}
<a {{ with $ref }} href="{{.}}" {{ end }} {{ with $target }} target="{{.}}" rel="noopener" {{ end }} class="book-btn{{ with .Get "class" }} {{ . }}{{ end }}">
  {{ .InnerDeindent | .Page.RenderString }}
</a>