The Unbeatable Squirrel Girl by Ryan North and Erica Henderson is one of my personal favorite series to come out of Marvel recently. However, it's also the most polarizing. For some people, it's just too silly and the humor just doesn't make sense to them, whole others find it the funniest book they've ever read. The art is equally unique and polarizing. I recommend you read the preview on Amazon to see for yourself.
Squirrel Girl also fills an important spot in Marvel's line. It is in no way a kids book, but it is perfect for kids, especially young girls.
Here's every Unbeatable Squirrel Girl book in reading order:
Squirrel Girl also has her own stand-alone graphic novel, which was released around volume 5. It's good on it's own, or alongside the main story: