Certainly the books have more detail about the crimes, plus more detail about the politics, both tribal and county. Longmire has a love life but keeps hooking up with the wrong woman, and getting hurt. Like I said, in many ways these books are a contemporary police procedural, kinda like a Western version of Robert B.Parker's Jesse Stone. The sparse prose is similar in style, and not at all like the historical Westerns that Parker wrote (Appaloosa and Monte Walsh).
For my money, the very best subplot that keeps popping up in the books is the recurrent one of the Indian spirits. In times of great physical stress and sometimes in dreams, Longmire actually sees and is aided by the ghosts of the famous Lakota warriors and Dog Soldiers. He will not even discuss this with Henry, knowing that if word gets around, he will never win re-election. In this aspect of Native American spirituality, the Longmire novels resemble those of Tony Hillerman. However, it would be very difficult to do this topic justice in a weekly TV series, whereas Johnson crafts each book over 12 months.
In the books, Vic the female Deputy is a fiery black-haired Italian going through a divorce in Montana, after she moved from Philidelphia to Montana with her new husband, giving up her job as a beat cop for 5+ years. Then he moved on to another job in Alaska, and she refused to go. I had doubts about Katee Sackoff in the role, but she's actually very good. I also had doubts about Lou Diamond Phillips as Henry Standing Bear, because in the books, Henry is a very large powerfull man and Phillips is slight. But Phillips is now a good actor, and portrays Henry well.
I highly reccomend the books, but I would not call them "darker", except that they unflinchingly display human weaknesses, as only an experienced lawman would see them.