Can you fit a Harry Potter book in it?? Or a comic????

Too small for a book. It's like a small coin purse/card wallet. Dimensions are 4.7 x 2 x 7.5 in

Will i be able to fit a phone in ( galaxy7), and a small wallet?

I own a Galaxy Note 3, and I was able to fit my phone in it. Closing the pocket took a slight amount of effort, but nothing to cry to mother about. ;)

Can an adult carry this around or is it just for children? Can it be used as a wallet inside a larger bag/purse?

An adult can use this, yes, as a wallet. It doesn't have much room, however. After your drivers license and a credit card, add a rewards card or two, it wouldn't be able to securely snap closed if you had any cash in it.

Can this clutch fit an epi pen?

It could fit the pen itself but not with its box. And that’s about all it could fit, there isn’t much space. Also, forwarning, the clip on mine broke after two days hah