My Roommate is a Cat

Subaru has begun living with a cat as a roommate. He realizes he's run out of cat food and goes to a pet shop to buy more, but starts to panic when he's surrounded by creatures he's never been around before. Then Nana, a shop employee, starts to talk to him, asking him questions about the cat's name and age. Subaru then realizes he hadn't thought of a name for the cat yet. Nana tells him that he should give the cat a name because it's his family now, so Subaru tries to think of one, but nothing good comes to mind. As he's pondering, he comes across a book in his room called "The Moon and the Sun"...