Fixed back button behaviour on books edit

As reported in #339

@@ -11,6 +11,6 @@
 </div>
 
 <div class="form-group">
-    <a href="{{ back()->getTargetUrl() }}" class="button muted">{{ trans('common.cancel') }}</a>
+    <a href="{{ isset($book) ? $book->getUrl() : baseUrl('/books') }}" class="button muted">{{ trans('common.cancel') }}</a>
     <button type="submit" class="button pos">{{ trans('entities.books_save') }}</button>
 </div>
\ No newline at end of file