Improved comments

Author: DavidLazarescu

src/application/interfaces/utility/i_book_getter.hpp

@@ -7,7 +7,11 @@ namespace application
 {
 
 /**
- * This is a basic interface to get a pointer to a book from any source.
+ * This is an interface to get a pointer to a book from any source, either the
+ * in-memory library or the file system. We need this because the book service
+ * needs to access the book to add highlights, bookmarks, etc. But it doesn't
+ * know if the book is in the library or on the file system (in the case of an
+ * external book). So it uses this interface to get a book pointer.
  */
 class APPLICATION_EXPORT IBookGetter
 {
@@ -20,4 +24,4 @@ class APPLICATION_EXPORT IBookGetter
     virtual bool bookIsValid() = 0;
 };
 
-}  // namespace application
+}  // namespace application