No one's mentioned Volokh's Academic Legal Writing yet - I hear it's stellar.
(How do you embed a link and give it a non-linky name, lke lackof did?)
I would not recommend reading this before 1L. Its focus is on academic ( Aka scholarly ) legal writing, which is very different in form, function and style from the writing you do for memos and briefs. Its also has its own different citation rules.
Seconded. Academic legal writing is a different beast than 1L Legal writing class.
In all seriousness though, I wouldn't even bother with worrying about legal writing class right now. Its a necessary evil, and you will learn everything you need to learn once you're in the class.
Thirded. (Which isn't a word, and if you think it is, there probably isn't any help for you). Take that advice and enjoy the summer.