DB Query not working in Plugin

Looking at the SQL query in $createSQL,

1. ENGINE=InnoDB" . $wpdb->get_charset_collate() . " AUTO_INCREMENT=1; yields to:

   Error in query (1286): Unknown storage engine ‘InnoDBDEFAULT’

   To fix it, add a space between the ENGINE=InnoDB (the storage engine) and " . $wpdb->get_charset_collate() (the charset).

2. You have defined the column id as AUTO_INCREMENT, but the column is not a key, and that yields to the following error:

   Error in query (1075): Incorrect table definition; there can be only
   one auto column and it must be defined as a key

   To fix it, define the column id as a key using either PRIMARY KEY (id) or KEY(id).

So, here’s the fixed code (or SQL query), tried and tested working on WordPress 4.9.8:

$createSQL              =   "
CREATE TABLE `". $wpdb->prefix ."_book_ratings` (
  `id` bigint(20) UNSIGNED NOT NULL AUTO_INCREMENT,
  `book_id` bigint(20) UNSIGNED NOT NULL,
  `rating` float(3.1) UNSIGNED NOT NULL,
  `user_ip` varchar(32) NOT NULL,
  PRIMARY KEY (id)
) ENGINE=InnoDB " . $wpdb->get_charset_collate() . " AUTO_INCREMENT=1;
";

If that doesn’t work for you, follow the guides here, such as use two spaces between the words PRIMARY KEY and the definition of your primary key.