Make Book Backlinks more selective

Books with simple titles sometimes had false positives if the word was
used in the text. This should make it so we only match on real links.

There is one edge case where it will match if you inlcude a
book_link.html but don't use the variable. I can't figure out how to get
around that. Yet.

_includes/book_backlinks.html

@@ -19,6 +19,12 @@ <h2 class="book-backlink-section">Reviews that mention <span
 {% capture newline %}
 {% endcapture %}
 
+{% comment %}To avoid picking up words in the text that match a book title, we
+instead look for the template we use to link to books, which should be
+unique.{% endcomment %}
+{% capture target_markdown %}book_link.html title="{{ page.title | downcase }}"{% endcapture %}
+{% capture target_url %}{{ page.url | downcase }}{% endcapture %}
+
 {% comment %}This sorted list will be used in a double for loop to insure that
 books are sorted in their sections{% endcomment %}
 {% assign sorted_titles = "" %}
@@ -49,9 +55,21 @@ <h2 class="book-backlink-section">Reviews that mention <span
 
     {% comment %}Check if the title of this page is contained in the other
     book's review. We have to used the normalized article text because if the
-    title is across a newline then it won't be found.{% endcomment %}
-    {% assign normalized_content = book.content | replace: newline, " " %}
-    {% if normalized_content contains page.title %}
+    title is across a newline then it won't be found.
+
+    At this point {{ normalized_content }} might or might not have been
+    rendered, so it could be either raw markdown or rendered HTML. We
+    therefore check for both the markdown link using {{ target_markdown }} or
+    the html link using {{ target_url }} raw markdown.
+
+    Also we don't have to worry about circular links because the content
+    variable doesn't include anything generated by the code in this file,
+    because this code is included in the layout, not in the markdown.{% endcomment %}
+    {% assign normalized_content = book.content | replace: newline, " " | downcase %}
+
+    {% if normalized_content contains target_markdown %}
+      {% assign placed_books = placed_books | push: book.title %}
+    {% elsif normalized_content contains target_url %}
       {% assign placed_books = placed_books | push: book.title %}
     {% endif %}