First published in 1954, set in 1963 and 1972, so as usual we can be slightly amused about predictions made about the future which is now the past. But actually this is a rather gripping book. There is a small group of Secret Guardians keeping watch over humanity's scientific and moral development (they happen to be from Mars but that is almost incidental). There is a renegade faction that wants to wipe out (most of) humanity to make Earth their own. For reasons not made completely clear, the conflict between the two focuses on a teenager in 1963 small-town Masachusetts and his piano-paying neighbour. There is a dramatic denouement halfway through the book, and we leap forward nine years, to the final struggle between the two in New York, against the background of unpleasant extremist US politics and biological warfare.
Yet the fundamental take of the book is very optimistic about human nature, and the message is a very moral one; not all fans are Slans. It almost reads like a 1950s reaction against today's libertarianism - perhaps I'm just not well enough read in the sf of the period - was this the height of van Vogt and Heinlein? I thought one peaked earlier and the other later. Interesting stuff, anyway.