What's the point of an enchanted book?

I've seen that you can have enchanted books in Minecraft with enchantments such as Sharpness and Knockback... but is there any point or advantage to beating someone to death with a book instead of, say, a sword?

The book is for enchanting items on the anvil.

To make an anvil you need 31 iron ingots. In a crafting table grid, place 3 blocks of iron across the top, 3 iron ingots across the bottom, and an iron ingot in the center:

 [] [] []
    --
 -- -- --

Then place the anvil, use it, place an item in the first slot, the book in the other. The enchantment costs Experience levels too (and you gain these gathering experience spheres by killing monsters, mining minerals, and smelting items.)

For example, a book of Sharpness III used on iron sword will make the sword cause 1.5 heart more damage on each hit.

You can enchant normal books too, using enchanting table - which might seem useless since the resulting book will have only one enchantment and you'll have to pay the exp cost twice (first, creating the book, and then again, enchanting the item with the book on the anvil). The serious advantage though is that the enchanting table gives enchantments randomly while the book gives the item exactly the enchantment of the book.

Imagine you want to make your diamond boots to have featherfalling enchantment. You can waste 40 diamonds on enchanting one pair of boots after another on the altar until you finally get the featherfalling - or you can use up a bunch of leather and paper until you get a book of featherfalling, then just apply it to a single pair of boots - 4 diamonds total.

Enchanting books and then transferring the enchantment to some other item via an anvil is usually superior to directly enchanting the item as it provides a buffer against the random nature of the enchantment system.

The inherent randomness of the enchanting system makes it impossible to guarantee a specific enchantment and specific enchantments (and even more so for combinations) can require many attempts to successfully get. This can lead to the generation of a large pile of (potentially) valuable items with unwanted or potentially 'useless' enchantments.

Another advantage of enchanting books and then transferring the enchantment is the ability to put enchantments on items that are impossible to obtain via directly enchanting the item (e.g. Efficiency V on a diamond pick, or silk touch on shears to them obtain cobwebs in survival mode).

The major downside to enchanting books is that they can take any enchantment available in the game, whereas items can only take enchantments that they can use. This means that getting a specific enchantment via enchanting books would be expected to take more attempts and therefore more XP than directly enchanting items. This effectively requires that you have access to an XP farm if you wish to get seriously involved in enchanting via books.

A enchanted book is like a portable enchantment. You just have to put the book and a item you want to enchant in a anvil, and the item becomes enchanted by the book's enchantment.

I suggest you read this.