Add demo for using enum to create singleton

src/main/java/feature/Book.java

@@ -0,0 +1,35 @@
+package feature;
+
+public class Book {
+    private String title;
+    private String author;
+    private int pages;
+
+    public Book(String title, String author, int pages) {
+        this.title = title;
+        this.author = author;
+        this.pages = pages;
+    }
+
+    public String getTitle() {
+        return title;
+    }
+
+    public String getAuthor() {
+        return author;
+    }
+
+    public int getPages() {
+        return pages;
+    }
+
+    @Override
+    public String toString() {
+        return "Book{" +
+                "title='" + title + '\'' +
+                ", author='" + author + '\'' +
+                ", pages=" + pages +
+                '}';
+    }
+    
+}