groupedByAuthorBooks = _.groupBy(books, 'author');
groupedByAuthorBooksKeys = _.keys(groupedByAuthorBooks);
console.log('\nAverage books price for each author:');

for(var i = 0; i < groupedByAuthorBooksKeys.length; i++) {
    var author = groupedByAuthorBooksKeys[i];
    console.log('Author: ' + author);
    console.log('Average books price: ' + (averageBookPriceByAuthor(groupedByAuthorBooks[author]) / groupedByAuthorBooks[groupedByAuthorBooksKeys[i]].length).toFixed(2));
}

function averageBookPriceByAuthor (author) {
    var totalBooksPrice = 0;

    for(var i = 0; i < author.length; i++) {
        totalBooksPrice += Number(author[i].price.replace(',', '.'))
    }

    return totalBooksPrice;
}