I agree with Mikaela. I once visited Recto to find a certain book that I need for my friend's birthday. You can try Booksales, too. It is my sister's to-go place whenever she wants to purchase something new to read. Another one is Cubao Expo. Certain stores there sells old books (even vintage comics and pocket books).
Booksales are good treasure troves for bookworms. You can find it anywhere in Manila. Usually, they're contain within SM malls. You may also visit Recto. There are certain spots there that offer good reads, hard-to-find and vintage books even.