const bookList = document.querySelector('#book-list');

console.log('#book-list next sibling:', bookList.nextSibling);
console.log('#book-list next element sibling:', bookList.nextElementSibling);
console.log('#book-list previous sibling:', bookList.previousSibling);
console.log('#book-list previous element sibling:', bookList.previousElementSibling);

bookList.previousElementSibling.querySelector('p').innerHTML += '<br />Too cool for everyone else!';