Buy from Bookseller NPC. He comes to Pelican Town twice a season and offers a different assortment of books at a high price.

Excavate in Artifact Spots. Look for worms in the ground, and they will show you the way.
Unlock Golden Mystery Boxes and Fishing Treasure Chests.

Keep progressing, and some NPCs will sell or give away books as part of the story.
Participate in all events, and you are sure to receive several books.

Some can also be spawned with the item code cheat.