It is rather a remarkable book. It's not totally clear that Marcus Aurelius wanted it to be published, or if this was basically his commonplace book (or books) for Deep Thoughts which his admirers circulated after his death. It's a bit jumbled thematically, so I'm inclined to the latter - I think he was a good enough stylist that he'd have organised it a bit better if he was interested in publishing it, and also I don't think he particularly was interested in publishing it. So we basically have the secret thoughts of the ruler of the Roman Empire at its height, which is really quite something. (Gibbon tells us, in footnote 47 to chapter III, that he actually gave public philosophy lectures, as Emperor, in Rome, Greece and Asia; presumably we have here some of the raw materials for those lectures.)
Marcus Aurelius was a believer in the Stoic philosophy: that one should accept one's lot in life, not worry too much about what other people think or about death, and just get on with doing as much good as you can given your personal circumstances. Of course, if your lot in life happens to make you the Roman Emperor, you possibly have fewer grounds to complain about it, or to worry about issues of personal status, than most people. But we all worry about death, including emperors. And Marcus Aurelius is not obsessed with his own celebrity or achievements; the first section of the book is a series of thank-yous to the influential people in his life for their wisdom and intelligence.
Sufficiently edited and bracketed with explanations, this could make a rather successful if somewhat unusual self-help book. It is not in the usual paradigm: rather than helping the reader look at their insecurities and work through and past them, Marcus Aurelius urges the reader (who in the first instance is himself) to put it all aside, reflect on the immense infinity of space and time, and just get on with it. In some circumstances that actually is the right advice. Though I wonder if even he was really convinced - was his recording of different material covering the same themes a matter of finding several different beautiful thoughts which appealed to him? Or was he trying to persuade himself by repetition?
Marcus Aurelius' biological legacy to the empire was his appalling son Commodus, whose reign Gibbon marks (in Chapter IV) as very much the crucial starting point of the decline of Rome. His intellectual legacy is rather more impressive, and certainly longer-lasting. I shall look out for a decent dead-trees edition of this; it is very much worth having on the shelves.