I've spent a fair amount of days at The Mountain without any trail guide book, but decided I needed one to get full enjoyment out of the park. The book has all the information you need to choose a hike based on highlights, season, difficulty, and solitude.
It is arranged by park region and even includes a few hikes from outside the park that have good views of Rainier. Directions on how to reach the hikes are clear, as are directions while on the trail. Maps are cropped USGS-style and show elevation gain well, along with adjacent elevation gain charts.
I appreciated the inclusion of autumn in some hikes' "best seasons to hike". Far too many books base this designation on wildflower seasons, but leaf color change and mushrooms can provide just as much interest in season. The author's sense of humor is pretty good, although forced at times. Regardless, it does not detract from the discussion f each hike.
All in all, I'd definitely recommend this book if you are only interested in day hikes.