This article goes into a fair amount of detail about how quantitative trading firms make money. It is usually difficult to find anything more than vague, surface-level information online about the industry. This is mainly because employees of these firms sign nondisclosure agreements to protect their firm's strategies. Also, specifics of the work quickly get into PhD-level mathematics territory. While the author doesn't go too deep into the intricacies of any strategy, there is a surprising amount of detail about the methods these firms employ.