Deadly Cool is a quirky, humorous murder mystery. The mystery is not obvious and even if you figure out the bad guy the humor and other elements of the story still make it very enjoyable. Hartley is a great main character with a level and humorous head on her shoulders, overall she feels very authentic. All of the characters are well written and feel like they could exist, except for maybe the investigating of the deaths. The tone of the book is lighthearted but still includes real emotion and tense scenarios. Nothing is terribly gruesome despite there being murders and attempted murder in the book, the details make it clear without being too horrific (not that it would bother me but such details wouldn't really go with the overall tone).
Overall, Deadly Cool is a fun read with a surprisingly good mystery. I look forward to other books in the series.