$by_key = $books->keyBy('title');
$by_key->all();
//result
/*
[
'Lean Startup' => ['title' => 'Lean Startup', 'price' => 10],
'The One Thing' => ['title' => 'The One Thing', 'price' => 15],
'Laravel: Code Bright' => ['title' => 'Laravel: Code Bright', 'price' => 20],
'The 4-Hour Work Week' => ['title' => 'The 4-Hour Work Week', 'price' => 5],
]
*/