The joke is that the Hardy Boys are for kids and 'True Detective' is for adults.
It's an obvious joke, but a good one. In fact, it's shocking that it took until two weeks after the first season of True Detective came to a close for someone to Photoshop Rust and Marty into Hardy Boys book covers. These are beloved American crime-solving duos, one from the past, one from the present. They juxtapose neatly: The Hardy Boys are clean-cut, wholesome young men who solve crimes usually ranging from petty to grand larceny, while the True Detective guys are world-weary alcoholic philanderers who solve quasi-Satanic rape-murders. Break.com's Todd Spence made these funny, well-crafted Photoshop creations. The results are almost too perfect.