I loved Fangirl and I loved Carry On too, but honestly I wouldn't say they had much of anything to do with each other -- Carry On reads pretty much like a very good Harry/Draco fic with a lot of worldbuilding, so if that's something you enjoy, have at it! It's not going to hit the same notes about anything in the real world, but it's a super enjoyable fast read. (And if you end up enjoying Carry On, I would put in a pitch for In Other Lands by Sarah Rees Brennan, which I think basically does a similar "I love this but let me just fix a couple things" take on Narnia.)