This is a flat slip bag for placing thin books, comics or magazines inside of. There is also an identical one where the graphics are in light blue instead of sepia, as shown here.
The quote on the bag says "Don't just sit there and waste your precious time. When you want to do something, do it when you can. Do it right away. It's the only way to live a life without regrets." This is a great quote as said by Sonic the Hedgehog. It appears on many Sonic products throughout Japan. It is quite sad that other countries didn't adopt it.

They come in 'standard' sizes, probably one to fit a book, and another to fit a small variety of magazines. Item is in the SonicGear Personal Collection.