Which version of the book? I believe there was an updated version released a couple years ago. The one I am referring to had lengths of 35 feet for shielded cable and 50 feet for unshielded cable. I am in the process of moving so I cannot find my copy at the moment. It is heavy on technical but the background information has actually helped me diagnose some problems in the past. I also enjoy reading about technology so I didn't find it as dry as some might. I don't remember my copy having any discussion of UTP or STP cables.

You are right that the spec doesn't state actual cable lengths but a formula for calculating cable lenghts. Newer UARTs can transmit further it seems but I still try to stick with the 50 foot marker. I also try to avoid twisted pair cable when possible. Ethernet has taken a lot of the serial control away so wire length becomes less of an issue assuming you stay within allowable wire lengths for Ethernet communications, which I think is 328 feet or so on copper. I haven't had enough coffee today to pull specs out of my head and I don't feel like Googleing for an answer. Regardless, I still prefer a soldered connection with proper serial wiring. It looks cleaner in the rack and more professional all around.