Add string representation of UserBook

reader/models.py

@@ -168,6 +168,9 @@ class UserBook(models.Model):
     # Last read
     last_read = models.DateTimeField(auto_now_add=True)
 
+    def __str__(self):
+        return f"{self.user} - {self.book}"
+
     def percentage_read_whole(self):
         return round(self.percentage_read * 100)