diff --git a/redstring/library.py b/redstring/library.py
index 30f38d7..6982937 100644
--- a/redstring/library.py
+++ b/redstring/library.py
@@ -2,7 +2,6 @@
 Logic for operations that depend on a whole collection of documents.
 """
 import os
-from pathlib import Path
 
 from redstring.parser import load, TagOptions, DocumentTag, TabOptions, DocumentTab, Document
 
@@ -11,16 +10,60 @@ def generate_index_document(directory: str) -> Document:
     """
     Generate a document describing a document collection.
     """
-    dirpath = Path(directory)
-    document_info: list = []
-    for filename in os.listdir(dirpath):
-        with open(dirpath / filename) as f:
-            document = load(f)
-        document_id = document.get('id')
-        title = document.get('title')
-        document_info.append((document_id, title))
+    categories: dict = {}
 
-    tag: DocumentTag = DocumentTag('Unsorted', str(document_info), TagOptions(), [])
-    tab: DocumentTab = DocumentTab('index', [tag], TabOptions())
-    doc: Document = Document([tab])
-    return doc
+    for filename in os.listdir(directory):
+        with open(os.path.join(directory, filename)) as f:
+            document: Document = load(f)
+
+        # Check if this document specifies a tab, and create it if necessary.
+        category = document.get_tag('category')
+        if not category:
+            category = 'index'
+        if category not in categories:
+            categories[category] = {}
+        category_tab = categories[category]
+
+        # Check if this document specifies a topic, and create it if necessary.
+        topic = document.get_tag('topic')
+        if not topic:
+            topic = 'uncategorized'
+        if '.' in topic:
+            topic, subtopic = topic.split('.', maxsplit=1)
+        else:
+            subtopic = None
+        if topic not in category_tab:
+            category_tab[topic] = []
+        topic_tag = category_tab[topic]
+
+        # Save the title and id.
+        doc_id = document.get_tag('id').value
+        if doc_title_tag := document.get_tag('title'):
+            doc_title = doc_title_tag.value
+        else:
+            doc_title = None
+
+        topic_tag.append((doc_id, doc_title))
+
+    # Build an index document
+    def document_link(info):
+        doc_id, doc_title = info
+        return (
+            f'<a href="/doc/{doc_id}">{doc_title} ({doc_id})</a>'
+            if doc_title else
+            f'<a href="/doc/{doc_id}">{doc_id}</a>'
+        )
+
+    built_tabs: list = []
+    for category in sorted(categories.keys()):
+        built_tags: list = []
+
+        for topic in sorted(categories[category].keys()):
+            docs = sorted(categories[category][topic], key=lambda x: x[0])
+            doc_links = map(document_link, docs)
+            value = '- ' + '<br>- '.join(doc_links)
+            built_tags.append(DocumentTag(topic, value, TagOptions(), []))
+
+        built_tabs.append(DocumentTab(category, built_tags, TabOptions()))
+
+    return Document(built_tabs)
diff --git a/redstring/parser.py b/redstring/parser.py
index 8c4b920..7e23d92 100644
--- a/redstring/parser.py
+++ b/redstring/parser.py
@@ -130,6 +130,12 @@ class DocumentTab:
         self.tags: List[DocumentTag] = tags
         self.options: TabOptions = options
 
+    def get_tag(self, name: str):
+        for tag in self.tags:
+            if tag.name == name:
+                return tag
+        return None
+
 
 class Document:
     """
@@ -141,7 +147,13 @@ class Document:
     def __iter__(self):
         return self.tabs.__iter__()
 
-    def get(self, name: str):
+    def get_tab(self, name: str):
+        for tab in self.tabs:
+            if tab.name == name:
+                return tab
+        return None
+
+    def get_tag(self, name: str):
         for tab in self.tabs:
             for tag in tab.tags:
                 if tag.name == name: