One way would be to use Collections#map:

$outputArray = $list
  ->pluck('title', 'id')
  ->map(function ($title, $id) use ($books_selected) {
    return [$title, in_array($id, $books_selected)];
  });

Or in a shorter form if you're using PHP 7.4:

$outputArray = $list
  ->pluck('title', 'id')
  ->map(fn($title, $id) => [$title, in_array($id, $books_selected)]);

Your final array should be in this form:

[
  id1 => ['title1', true],
  id2 => ['title2', false],
  ...
]

source: StackOverflow