Yeah, it's a prequel, one of the last written. I usually approach these things under the assumption that, for instance, anyone who knows Magician's Nephew comes first but was published later will know the series well enough to get the question anyway. It's sort of like if I asked "This book, published in 1990, was the first in a series which has neither a beginning nor an end, and begins many years prior to the events of the main story." Anybody who recognizes that series and sees how EotW and NS both satisfy it probably will know or be able to guess which one was printed in 1990. Prequels are always tricky when referencing 'first in a series' though, especially since virtually every major trilogy and series has some sort of prequel including most of the ones on the list.