Kettlebell circuits are the best thing i have done exercise-wise in a long time.
I started by going to a class, but i quicky found it was more orientated towards middle-aged ladies so i changed to another trainer and his classes are ace. He explains the routines along with stressing the proper posture throughout.
At home i use The Ultimate Kettlebell Workbook from Dave Randolph.
Again, it has different routines for varying levels and what you are trying to achieve along with well explained and photographed exercises.
Love it!!!
Using mostly a 16kg at the moment, that fits well with me as im not after low rep strength work, mainly high rep intervals.