A book I like a lot is <i.The Quaker Book of Wisdom</i> I've recommended it a lot, but interestingly, to different people for different reasons. It has a section on absolute truth-telling (which I hadn't known was a Quaker principle), and has a good discussion on things like lying by omission or by implication (one example story he tells is about 'regifting', and whether it counts as a lie or not).