You can find books, notes and treasure maps all over the game. Places you can find them include:
- Notice boards
- Bookshelves (both waist-high and large)
- From certain merchants
- As a book or note displayed on a desk
- From certain quests.
You will find them in your inventory under both the 'Usable Items' and 'Quest Items' categories. Simply press the A button to read one.
*Some items disappear from your inventory after reading them, so having 30 at once is not necessary.
If you loot bookshelves and visit notice boards this should be obtained by roughly level 4-6 without having to go out of your way to search for them, so relatively early in the game.