When should I take the Spanish subject test?

<p>My mom decided today that she wanted me to take a math sat II and a spanish sat II. I've heard of subject tests before but I really know very little about them. But right now I'm in honors spanish IV with a 95 in that class and I plan of taking AP spanish language next year. My dilemma is that I don't know whether I should take it sometime within the next few months or should I just wait until my AP class to take it.
Also, are there any kind of prep books you'd recommend for these 2 subject tests?</p>