This is my favorite Heinlein novel, and I've read all of Heinlein's works. It is a great mixture of adventure, humor, politics, technology, some thought provoking looks at alternate types of marriages, and the most lovable sentient computer ever to grace the pages of a novel. Mike (the computer) is really the star of this book, from loving to tell jokes, to deciding to help a group of revolutionary-minded Luna 'citizens' actually accomplish their dreams of freedom because the human interaction would keep him from being lonely.
Along the path to revolution, Heinlein, (as usual), inserts thoughts and ideas that challenge your basic assumptions about what is right, normal, necessary, or appropriate. Is a representative democracy the only 'good' form of government? What's so sacred about a 'majority'? How should a government finance itself? (Maybe make the representatives pay for their pet projects out of their own pocket - taxes not allowed!). Are polygamy, polyandry, or other forms of multiple marriage wrong or can they be used to help preserve the stability of a child-rearing environment? How do you most efficiently organize a revolutionary group that must be kept secret from the authorities (given the assumption that there will always be 'stool pigeons')?
Some have quite correctly noted that this book should not be read by ultra-grammarians, as it is told in first person Luna-speak, an odd pidgin mixture of English and Russian, with occasional items thrown in from Chinese, Finnish, and several other languages. Far from being a detriment, I consider this to be a great accomplishment. Most writers have trouble accurately portraying the dialect, say, of the Deep South in a convincing manner. Here, Heinlein has created his own dialect of the future - and makes you believe it.
This book is not quite as deep as Stranger in a Strange Land, one of Heinlein's other great books, but it has a faster, more action oriented pace, and characters that you will get emotionally involved with. I cried at the end of this book the first time I read it (and the second, and the third...) and I think you will too. TANSTAAFL indeed - but in this case, you get more than you paid for.