The Mickelson trail guidebook is an excellent source of information. Available all over south Dakota or online.
I have ridden it twice in the spring, but I usually hotel it for accommodations. The highlight reel as I like to call it is from Hill city to Mystic or Rochford. If you like a midday beer the Moonlight gulch Saloon in Rochford is a neat spot. From Hill city to Mystic you get a good feel for the old bridges, the grade and two old tunnels. It makes for a nice round trip if you are inclined to go that route, which is how I first started to ride the trail while up there on business, one round trip section at a time. Lots of good info on line also,
Mickelson Trail Affiliates.
Good luck and have fun, it is my favorite rails to trails trip.