diff --git a/include/book.h b/include/book.h
index b5ff34052..d53de75d4 100644
--- a/include/book.h
+++ b/include/book.h
@@ -42,10 +42,16 @@ class Reader;
 class Book
 {
  public: // types
-  struct Illustration
+  class Illustration
   {
+    friend class Book;
+   public:
     std::string mimeType;
     std::string url;
+
+    const std::string& getData() const;
+
+   private:
     mutable std::string data;
   };
 
diff --git a/src/book.cpp b/src/book.cpp
index ed219ce79..8c992deac 100644
--- a/src/book.cpp
+++ b/src/book.cpp
@@ -226,16 +226,20 @@ Book::Illustration& Book::getMutableDefaultIllustration()
   return m_illustration;
 }
 
-const std::string& Book::getFavicon() const {
-  const Illustration& favicon = getDefaultIllustration();
-  if (favicon.data.empty() && !favicon.url.empty()) {
+const std::string& Book::Illustration::getData() const
+{
+  if (data.empty() && !url.empty()) {
     try {
-      favicon.data = download(favicon.url);
+      data = download(url);
     } catch(...) {
-      std::cerr << "Cannot download favicon from " << favicon.url;
+      std::cerr << "Cannot download favicon from " << url;
     }
   }
-  return favicon.data;
+  return data;
+}
+
+const std::string& Book::getFavicon() const {
+  return getDefaultIllustration().getData();
 }
 
 const std::string& Book::getFaviconUrl() const