In the past I've made a number of observations about opening study, but one thing that hasn't been discussed is studying your chosen defense from the other player's perspective. When you have limited time to put into your chess studies, it makes sense to focus on books and other materials that treat your defense (the Caro-Kann, the Dutch, etc.) from Black's perspective. Ideally they will be both objective and comprehensive. The best indicator of this is how White's plans and prospects are treated.
Too often, especially with resources aimed at the Class player, Black's chances are exaggerated, or White's are downplayed (which amounts to the same thing). It's interesting to note that so-called repertoire books, while narrowly focusing on Black's preferred moves, tend - if the author is honest - to provide a deep look at all of White's possibilities. This is made easier by limiting the scope of the material covered and subjecting the chosen variations to (hopefully) rigorous testing. Black players benefit tremendously from making a serious effort to fully understand all of White's plans and how to respond to them.
Taking the idea further, I've found some of the most valuable opening study material for my defenses to have been written from White's point of view. The Caro-Kann in Black and White, for example, was authored by Beliavsky and Karpov, each taking one side's perspective; Beliavsky's portion (as White) taught me a lot more. A more recent example that looks worth examination by Black players is 1. e4 versus the French, Caro-Kann and Philidor, reviewed by GM David Smerdon on Chess.com. It's always good to see the other side's playbook, especially when it deepens your own understanding.