<input name="book_active" id="book_active" type="hidden" value="N">
<input name="book_active" id="book_active" type="checkbox" role="switch" class="form-check-input" value="Y">
<script>
function edit_book(id) {
save_method = 'update';
$('#form')[0].reset(); // reset form on modals
$.ajax({ //Load data from ajax
url: "<?php echo site_url('book/ajax_edit/') ?>" + id,
type: "GET",
dataType: "JSON",
success: function(data) {
$('[name="book_active"]').val(data.book_active);
//zaznaczenie checkboxa jesli book_active=Y
if (data.book_active == 'Y') {
$('#book_active').prop('checked', data.book_active == 'Y'); //.prop (name, value)
}
},
error: function(jqXHR, textStatus, errorThrown) {
alert('Error get data from ajax');
}
})
}
</script>

if ($('[name=book_active]').prop("checked") == true) {
$('[name="book_active"]').attr('value', 'Y');
} else {
$('[name="book_active"]').attr('value', 'N');
}