The best place for current conditions of all the trails and back country campsites at Mount Rainer may be found at this National Park Service page.
The definitive book about the Wonderland Trail is Bette Filley's: Discovering the Wonders of the Wonderland Trail: Encircling Mount Rainier. Multiple editions exist. If someone asks you if you have "the book" about the trail, this is the one they are talking about.
