I came across a sentence recently: Before I turn 40, I want to have written a book.

Could someone explain to me what does it actually mean?

I'd rather say: Before I turn 40, I want to write a book. but I'm not native speaker.

I understand the meaning of PERFECT INFINITIVE in sentences below:

If I had known you were coming I would have baked a cake.

Someone must have broken the window and climbed in

but I can't grasp the difference between:

Before I turn 40, I want to have written a book vs Before I turn 40, I want to write a book

Thanks for any help Marcel