Roderick Graham 
Kirjath and the Earth Warriors [EPUB ebook] 

Brothers Michael and Gabe are magically transported across the universe to Kirjath where hundreds of aliens from all over the universe gather to fight an incoming space-fleet. The boys must live with, train with, and eventually overcome many of them to ensure their own safety. They quickly realise that someone, or something, on the planet is helping them. On this strange planet of monsters and magic, where Earth is is just a myth, they must find inner strength if they have any chance of getting home again.