Java Edition:A Book
A Book is a book that was obtainable in the April Fools' 20w14∞ update. The discontinued part is the author tag "Developer". However, there is a player called Developer, therefore it is only technically discontinued in singleplayer (unless you are Developer), or the server has whitelisted without, blacklisted, or banned them.
Obtaining
Original
The "llama" dimension is an easter egg dimension that brings the player to a room with a llama. In the corner of the room with the portal and window, there is a hidden chest containing the book (see picture). There are many rooms in the dimension and more can be obtained by going to different rooms.
Copies
Written books may be copied in 13w36a (a 1.7.2 snapshot) and above by crafting them with a book and quill. 14w05a (a 1.8 snapshot) added the generation tag, which increases by one each time the book is copied. The three variants that can be created are "Original", "Copy", and "Copy of a Copy". Books that are a copy of a copy cannot be copied further. All books created before this point and newly signed books will be considered original.
Gallery
A Book with Developer as it's author
The inside contents of A Book
The chest hidden in the corner of the llama room
See also