Alastor is an unlinked trilogy, three novels with the only common factor being the "Alastor Cluster", a sector of space more or less ruled by a "Connatic", subject to the various cultures of a thousand different worlds. We have three of the worlds on show here, and each is the setting of a different mystery.
Trullion is a watery world, with man and merling in constant low-level war: there is a truce of sorts, but it boils down to a merling on land, or a man in the water, is fair game to the other. On this world, the mystery is the murder of Glinnes Hulden's father and brother, and the dissolution of his inheritance. This provokes Glinnes to action, seeking to raise money to reclaim his ancestral property. Unusually for a Vance novel, this is by way of a sporting contest - the intricate game of Hussade, about the only thing that appears to interest the lazy and decadent "Trills". Eventually the mystery is solved - not that it is terribly difficult - but other problems arise, including those relating to a pretty girl. This is a gripping story, with a fair degree of depth to it: it is Vance, after all.
Marune, on the other hand, presents us with an amnesiac hero dealing with political meddling - and, as it turns out, he also must reclaim a stolen patrimony and avenge a murdered father. So there are some thematic similarities to Trullion, but that's about it. We have here a wonderfully strange society on a world where the suns rarely set, and where eating and drinking are treated as we would treat the evacuation of waste flowing from said eating and drinking. As you might expect, sex is also off the menu: until the rare nightfall, anyway.
This society - and all of humanity's quirks and foibles, by extension - are examined here. As well as that though, there is fun story being told, and if the villains are perhaps a touch cartoonish, well, that's a small price to pay.
Wyst is a mystery in that it opens with the Connatic querying the fate of a "Jantiff Ravensroke", which we then spend the balance of the book discovering. Jantiff is an offworlder who has traveled to the egalist world of Wyst, where everyone works only 10 hours a week, eats nothing but reprocessed waste and dead people (although nothing is made of that by the characters), and as a result everything is half-broken and the inhabitants steal everything they can from everybody else - after all, why should anyone have more than anyone else? Its not fair otherwise.
This may be easily read a critique of Socialism, but its probably a critique of human nature (which, in turn, is the heart of the problem with Socialism - and every other political system). In this egalist paradise there is a nasty plot underfoot, and Jantiff is somehow pivotal to its success or failure. But really, almost the best thing about this story is the love and desire for "bonter" - ie fresh real food, not just "gruff and deedle, with wobbly to fill in the cracks". The lengths that people will go to to get it is amazing.
All of Alastor is fun, all is readable. It also forces you to think. The plot points I have sketched out above are by no means complete, and with Vance the telling is at least as important as the tale - but luckily both are superb.