Is this the most useful book series ever? Since 1976, Sage has published over 170 little green books. More formally known as the Quantitative Applications in the Social Sciences (QASS), this book series provides short and accessible texts on all kinds of quantitative methods. Need to know more about fixed effect models or factor analysis, look no further!
Given their concise nature, the little green books are quite affordable, especially when compared to specialist treatments of methods more generally. Despite the growing number of online tutorials, these books still have a role. I tend to find them more comprehensive and balanced in coverage, and often more accessible.
Unfortunately, only very few of the books are updated — this seems to be at odds with the claim to be cutting-edge. There is no easy way to get the books online, except some of the most recent ones (Google Play seems to have most of them as scanned PDF, though). At a time when competitors like Springer sell (methods) chapters online, this is something hard to understand. The little green book offer a quick introduction and offer help to researchers with specific questions; this is why they probably want to be accessed instantly. That said, the little green books, like many academic books, may be targeting libraries more than individuals. A PhD student may not find $19 so competitive, especially when there are good online tutorials and Stack Overflow out there.