Captain Underpants Book Series by Dav Pilkey

Each book is $5 - The Adventures of Captain Underpants (Really bent) - Captain Underpants and the Attack of the Talking Toilets - Captain Underpants and the Invasion of the Incredibly Naughty Cafeteria Ladies From Outer Space (and the Subsequent Assault of the Equally Evil Lunchroom Zombie Nerds) - Captain Underpants and the Perilous Plot of Professor PoopyPants - Captain Underpants and the Wrath of the Wicked Wedgie Woman