There are 2 books in the The Unbeatable Squirrel Girl (2015) series. The newest book is Squirrel You Know It's True which came out in 2015.

1
Squirrel Power

By Ryan North , Erica Henderson (illustrator) ,

Book cover of Squirrel Power
Collects The Unbeatable Squirrel Girl #1-4, Marvel Super-Heroes (1992) #8.

Doctor Doom, Deadpool, even Thanos: There’s one hero who’s beaten them all — and now she’s starring in her own series! That’s right, it’s SQUIRREL GIRL! The nuttiest and most upbeat super hero in the world is starting college! And…
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2
Squirrel You Know It's True

By Ryan North , Erica Henderson (artist) ,

Book cover of Squirrel You Know It's True
Collects The Unbeatable Squirrel Girl #5-8, GLX-Mas Special #1 (first story), The Thing #8 (pages 1-8), Age of Heroes #3 (Squirrel Girl story). The breakout character of 2015 continues her one-woman crusade against injustice, criminals and jerks! Squirrel Girl meets potential new allies including Chipmunk Hunk, Koi Boi and…Girl Squirrel?!…