Apple’s Books store hasn’t supported gifting for quite some time. You can now get it for your iPhone or iPad as well as your Mac PC.
Using Mac App: Find the book you want to give as a gift, click the drop-down menu in the button that displays the book’s price, choose the “Gift This Book” option, and follow the on-screen instructions.
Using iPad/iPhone App: Use the iPhone or iPad app for Apple Books to send a book as a present; once you’ve located the book, hit the “Gift” icon (it looks like a wrapped gift box) that appears under the cover.