allow books to stack 4 at a row

extensions/book/public/book_style.css

@@ -2,7 +2,7 @@
     width: 226px;
     height: 346px;
     display: inline-block;
-    margin: 0 12px 20px;
+    margin: 0 2px 20px;
     box-shadow: 2px 4px 16px rgba(0, 0, 0, 0.12);
     position: relative;
     vertical-align: top;