While it is true 0Ls don't know the language because they haven't prepared, when I wanted to learn Spanish, I moved to Spain for a year. I didn't pick up the language from "Spanish for Dummies." Likewise, law school is complete and total immersion. You will learn the language there, by reading cases and listening to professors. It is a system that has proven to be consistent. Don't freak out now. If you want to read a book about how to pass your first year, go for it, but it isn't necessary, and thousands of students have been successful in law school without it.
While it's always true that we tend to see what we want to see, in my mind this example shows just the opposite: how much more effective is it when learning a foreign language but to take even a few minutes to read a book about the rules of that language? It doesn't seem too radical to state that the cost-benefit is near infinity on the plus side--and that a decision to buy the ticket to Madrid and to heck with those sissy books is, well, a cost-benefit approaching null.
Even children--whose minds are uniquely pre-wired to absorb language--tend to hold back in new-language environments. If you've seen this in action, you'll even see their hesitation, before they (eventually) barge ahead. (And when they do barge ahead, even if they don't hesitate, it's often chatting happily away in their own language, oblivious to the disconnect.) More than that, we tend to forget that it take a few years to get even the basic rules of grammar down. In law school, you've nine months.
Immersion--which is law school too ( . . . but it doesn't have to be)--is the steak and potatoes, per chance with a side of mejillones en vinagreta de tomate y langostinos cocidos, si como no? But taking a meal without an aperitif?! Perish the thought, old chap. = : )