Use orderedMap for language books

lib/models/book.js

@@ -161,6 +161,20 @@ Book.prototype.isMultilingual = function() {
     return (this.getLanguages().getList().size > 0);
 };
 
+/**
+    Add a new language book
+
+    @param {String} language
+    @param {Book} book
+    @return {Book}
+*/
+Book.prototype.addLanguageBook = function(language, book) {
+    var books = this.getBooks();
+    books = books.set(language, book);
+
+    return this.set('books', books);
+};
+
 /**
     Change log level
 

lib/parse/parseBook.js

@@ -39,10 +39,7 @@ function parseMultilingualBook(book) {
         .then(parseConfig)
         .then(parseBookContent)
         .then(function(result) {
-            var books = currentBook.getBooks();
-            books = books.push(result);
-
-            return currentBook.set('books', books);
+            return currentBook.addLanguageBook(lang, result);
         });
     }, book);
 }