Read a book when I was 9 or 10, its infuriating me that I can't find it.

The book in question I remember pretty much nothing about. It was part of a book series which I can't remember the name of. It started with an E I think. The cover of one of these books had a lot of green on it, had a blonde (I think that was her hair color) girl looking down at...something. Can't remember what. The main enemy was a sea creature I think, a squid like thing. I remember at one point in the series the characters were on a tower like island that towered out from an ocean and they were attacked by the enemy squid thing. Thats all I know. Thanks for the help.