Your local library should have some, or online.
In the present, you can find books or films based upon Greek Mythology.
Poseidon is a figure from Greek mythology, so you can learn more about him by reading ancient Greek texts like the Iliad and the Odyssey. You can also find information about Poseidon in books about Greek mythology, or by exploring websites dedicated to Greek gods and goddesses.
All the stories of the Greek gods and their doings fill big books. Ask at your local library!
Literature
because he goes on the internet and reads books on greek mythology
Homer was a blind poet who wrote the Odyssey and the Iliad. Both of these books had bits and pieces of Greek mythology in them.
Medusa is a character of Greek mythology. Her story, as well as many other stories of Greek mythology can be found in various books and websites. A few books that contain information on Medusa include 'Perseus: The Hunt for Medusa', 'Medusa,' and 'Sweet Venom.'
Amazon.com has a wide selection of books on Norse Mythology.
(The title really is just Mythology)
There are books which more often collect a handful of Greek Mythology; you'd have to look up the name "Artemis" to find out if there are any out near you about her alone.
bunch of books, especially by rick riordan
The chronological order of Rick Riordan's Greek mythology books is as follows: "The Lightning Thief," "The Sea of Monsters," "The Titan's Curse," "The Battle of the Labyrinth," and "The Last Olympian."