Adding data to node object
hook_node_load() is exactly what you're looking for

Act on arbitrary nodes being loaded from the database.
This hook should be used to add information that is not in the node or node revisions table, not to replace information that is in these tables (which could interfere with the entity cache). For performance reasons, information for all available nodes should be loaded in a single query where possible.

The code example from the docs page is actually very good for your use case so I'll include it here too:
function hook_node_load($nodes, $types) {
  // Decide whether any of $types are relevant to our purposes.
  if (count(array_intersect($types_we_want_to_process, $types))) {
    // Gather our extra data for each of these nodes.
    $result = db_query('SELECT nid, foo FROM {mytable} WHERE nid IN(:nids)', array(':nids' =&gt; array_keys($nodes)));
    // Add our extra data to the node objects.
    foreach ($result as $record) {
      $nodes[$record-&gt;nid]-&gt;foo = $record-&gt;foo;
    }
  }
}