Harris goes into what he means by lying and the shades of lying people take. From deception, white lies, twisting the truth, faking praise etc.. all of it he concludes you should no do. Perhaps if you were a spy during war time, then it might be OK to lie, but most of us do not live in that specific scenario. Harris points out that lying by definition is not cooperating with others, so it detrimental to society. Even telling someone that something they wrote was "good" or "nice" when really it wasn't doesn't help the author whose feelings you do not want to hurt. Instead of faking praise, you could offer constructive criticism, and help them improve their work, or save them the time and effort pursuing something that they are just not good at.
Of cause Harris is an atheist, so his motivation for not lying is not based on some divine command. Instead it is more of a pragmatic reason: lying does not help you or society. While I agree with the horizontal aspect of not lying, there was clearly lacking a vertical or moral absolute reason for not lying in this book. He says don't lie as practically it will go well for you if you tell the truth, not because it is in some objective sense "wrong" (whatever that means).
Harris concludes that we should all live lie free posting these questions to the reader:
How would your relationships change if you resolved never to lie again? What truths might suddenly come into view in your life? What kind of person would you become? And how might you change the people around you?
It is worth finding outBy which I take Harris to mean that it is possible that we are able to choose to lie or not to lie, which I agree with. But Harris has written another short book on Free Will which says otherwise...
You would think that we should therefore not punish people for their actions, as after all people are not in control of themselves, we all just act on what our body is telling us. But when a bear or wild animal runs around suburbia doing what it naturally does (like mauling people for food), it has to be restrained for the good of society regardless of its natural promptings. So, like lying, pragmatically we should punish people who by share chance and not fault of their own are naturally prone to anti-social behavior like rape and murder, for the good of society.
Harris tries to say that free will is not true, but knowing this still makes us better people not worst, this is because we can understand people better and see that their actions are really causes from their past or biological makeup. Of cause Harris ideas about free will are caused by his own past and biological makeup and who really knows where his argument really came from... the same could now be said about his atheism... (Don't blame me for that snarky comment, I am just doing what the atoms in by body is telling me to do; and if you get upset by that, then you are only doing what the atoms in your body us telling you to do. We are even... but not free).
Harris leans way too much on biology and not so much on historical (philosophical?) arguments about free will. I think if someone was going to write a book about free will they perhaps should look at the historical arguments for and against it and not just the biological science of the brain. Also if I was going to write anything on this topic I think a quick scan over Johnathan Edwards Freedom of the Will might be in order before saying anything. I hear it is quite good.
If I am to be really honest, I found Harris' book on free will quite lacking, and though it undermined everything he wrote in his book on lying, because it turns out I can't stop myself from lying or from telling the truth. My mind just throws up desires and ideas, but somehow Harris has risen above this and told us how the world truly is, all the while using his own biological brain that throws up desires and ideas at him which he has no idea where they came from such as picking beer over wine, or sometimes having a tea instead of a coffee in the morning.
Both these books are short and easy to read. The big problem I found with them is that you can not read them together. So perhaps you should just pick one... if you can.
Doug Wilson has also written a chapter by chapter response to the free will book, which is really quite a good read.