The Unbeatable Squirrel Girl is hilarious and fun, and one of my personal favorite series currently running at Marvel. So fun actually, that for some people, it's just too silly, but others find it the funniest book they've ever read. The art is also a little too different for some people at the beginning, but it matches the book so perfectly you'll grow to love it.
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.
The character Squirrel Girl has a longer history at Marvel, but you should absolutely start with her wildly popular first ever solo series, The Unbeatable Squirrel Girl, which is still being released.
Here's every Unbeatable Squirrel Girl book in reading order:
Squirrel Girl also has her own stand-alone graphic novel, which was released around book 5. It's good on it's own, or alongside the main story: