DDD add entity to aggregate: formed entity vs parameters

Question

which approach should be taken when adding entities to an aggregate?

I could add an constructed entity to the aggregate

$book = new Book($title, $releaseDate, ...);
$library->addBook($book)

or I could pass the parameters to the addBook method and let the method to construct the instance:

$library->addBook($title, $releaseDate, ...);

Answer 1

I would take the:

Library.AddBook(Book book)

Approach. Essentially a Book can exist outside of a library and we would want to add a particular book instance to a library, not a duplicate instance.

I think this would be the general case for child objects, although you can imagine cases where the child object is an integral part of the parent and the second method might be preferable. Maybe...

Baboon.AddHeart(parameter list...)

Here you can't add just any heart, there will only ever be one etc. its more like a setup step in creating a Baboon than adding a child object.

var b = new Baboon(name: "Jimmy")
    .AddHeart(parameter list...)
    .AddLungs(parameter list...)
    .AddLiver(parameter list...)
    .AddBrain(parameter list...)
    .Vivify();