I will say I was basing my previous "book value" estimate on the wrong industry. My original estimate was using the industry metrics for the "Industrial Metals and Minerals" industry.
I just took a look at the industry that SQM is categorized in on Yahoo Finance which is "Chemicals-Major Diversified".
Given that information instead of a Price to Book multiple of 5.5 we could potentially be looking at a Price to Book multiple of 7.8 http://biz.yahoo.com/ic/110.html