As far as literature goes, the older the book the better. This is not necessarily true for the content, but it is certainly true for the reader's ability to sift through quality ideas (which are universal) vs. culturally influenced trends.
While perusing John Calvin's Institutes of the Christian Religion a 21st century reader can easily discern between content that is solidly biblical (of which there is much) and content that is loaded with emotional baggage regarding Catholicism or the Anabaptist movement. While this doesn't mean Calvin's points concerning differing theologies are illegitimate, our cultural distance from the author enables easier detection of the his faults.
I am less able to do this with recent North American literature because I likely share the cultural and emotional starting point as the modern author. I see many of the same issues with evangelical Christianity (consumerism, apathy, biblical illiteracy, over-contexualization, etc.) and so it's likely that I have already arrived at similar conclusions.
A prime example of this can be found in one of the foundational documents of our denomination: the Heidelberg Catechism. You're reading along and when you get to Q&A 80 (which goes out of its way to bash the Catholic mass) and you realize the catechism has overstepped its purpose. On the other hand, the relevance of the rest of the catechism and the other reformed confessions shows their Spirit-led authorship and, in my view, means they succeeded in remaining close to the eternal and universal words of Scripture.
Perhaps the biggest reason I love to read old books is that it makes me feel connected to the Church. The giants of theology dealt with the same things I face as a pastor. Augustine was surprised at the amount of grief he felt when his friend died. Aquinas preached to people who were religious but not holy. Calvin struggled to continue his journey because of setbacks and controversy. These aren't modern problems. They're human problems. And there's a reason the answers these guys gave are still being taught in classrooms.
"What has been will be again, what has been done will be done again; there is nothing new under the sun. Is there anything of which one can say, "Look! This is something new”? It was here already, long ago; it was here before our time." - Ecclesiastes 1:9-10
