Written Book

Written books are created after a book and quill is signed. In the inventory, written books have a purple hue (similar to enchanted items) when obtained. After it had been signed, it can not be edited again (except if you use hack programs like NBTedit). The label does not say "Written Book", but what the player titles it, unless the item is spawned in through /give or with an inventory editor. It will say the title on the top line of the label, and "by username" on the bottom. Username is the username of the player who wrote the book, or "Player" if signed while playing offline. The icon of the written book is slightly different from the book and quill's icon, showing the same red book, but without the quill. The contents of a book are an extra set of data attached to the item. This means that when a book is destroyed, its contents are lost with it (unlike the case with maps).

When written books are obtained with /give or inventory editors, the label will only show "Written Book" and will be blank when opened. The book is signed and cannot be edited.

Trivia
In Weekly Snapshot 12w22a, you can trade written books for emeralds at 1:1 ratio. This makes a "publishing business" in Minecraft highly profitable.