Node access example

mymodule.module

function mymodule_node_access($node, $op, $account) {
  $controlled_nids = array(
    1,
    2
  );

  if (isset($node->nid) && in_array($node->nid, $controlled_nids)) {
    $acceptable_users = array(
      '2',
    );

    if (!in_array($account->uid, $acceptable_users)) {
      return NODE_ACCESS_DENY;
    }
  }
}