I'm starting this review with one small request: buy the book. You will not be disappointed, and, like me, I think this is one that won't just sit on your bookshelf. The art is fantastic, and the quality of each small story is pretty good, actually. So, please consider it. It's between 10-15 US dollars.
That being said, Time Killers is an awesome collection of short stories that, through out a few of them, you can see characters that resemble some of Kato's characters from Blue Exorcist (i.e. Astronerd, The Miyama-Uguisu Mansion Incident).
There are at 3 small stories that are colored in gouache (Usaboy!!!, A
Warrior Born of Red Earth, and a Guide to Princess Clothes), and they are beautiful. A Warrior Born of Red Earth is probably my favorite of the really short stories, as it tugs at the heart strings (oh boy, does it).
Her earliest one shot, Me and the Rabbit (Boku to Usagi) is a good one as well, and you can definitely see her improvements from that point. However, my favorite over all is a tie between Tomato and The Miyama-Uguisu Mansion Incident. You'll see why.
So, please read. It's definitely a book worth your time.