For those who are facing a career or job change, I’d suggest starting with an indentification of your own personality profile, skills and passions. A couple of ways to get this information are described here. You can also turn to the ever-useful What Color Is Your Parachute? for tools that will uncover this kind of information.
If you’re interested in becoming an expert in a field, use Ed Horrell’s books and weekly emails. They are tops at teaching you how to begin writing, speaking and offering consulting advice in a field of your choosing.
I’d also recommend Dan Miller’s excellent 48 Days to the Work You Love web site and materials. These are particularly useful to those who definitely want to go to work for someone else.
Po Bronson’s book is less a workbook and more a thought-provoking analysis of how others faced the decision described as What Should I Do With My Life?