Gensler, prof . of philo. at John Carrol U., has written here a book that is both very clear, formal and yet appealing as each chapter begins with the thoughts of the same fictional character and then deals with them. Gensler refers also to some online exercises he has put on his site. What may be a limitation with the book is that it is mostly limited to the question "how do we know what to do?" (which is extensively covered by a survey all the theories). This is how I understand why Gensler rejected supernaturalism (divine command). If he had dealt with the reality of right and wrong, then he would probably have admitted that without a purposeful mind beyond the cosmos there would be no right/wrong, that it would not matter if all humans would disappear in awful sufferings, or whatever. In his synthesis (final) chapter, Gensler seems to advocate the golden rule, but the reader may be given the impression he advocates a kind of situation ethics , because of another issue that was again not really dealt with (conflict between duties, etc.).
For a more systematic (but dryer and more difficult) work, I would recommend Norman Geisler's Introduction to philosophy or his Ethics. Anyway I found Gensler's book excellent. A more fun and less formal intro. is Steve Wilkens' Beyond Bumper Sticker Ethics.