This trilogy was first published as three individual novels: Northworld (published 1990), Northworld Vengeance (1991) and Northworld Justice (1992), although I have all three in one paperback omnibus, published by Baen in 1999. The first novel (but not the others) has the distinction of its own Wikipedia page, so if you want a thorough plot summary – complete with spoilers – you can look it up.
The principal character of the story is Nils Hansen, a classic SF hero; an intelligent and highly capable leader of a special police unit on the planet Annunciation, and exceptionally skilled in close combat. His planet is one of 1,200 settled by humanity (alongside their androids), all joined in the all-powerful Consensus of Planets. Hansen has just successfully concluded an operation in which he managed to arrest the head of a major criminal organisation, when he is abruptly recruited by mysterious agents of the Consensus for a special mission.
Hansen is told about the discovery of a planet suitable for occupation, but successive colonisation missions have disappeared without trace – and now the planet (named Northworld after an expedition leader) has vanished also. Hansen's mission is to travel to the last known location of Northworld to try and find out what is going on, the hope being that one man in a small vessel might attract less attention.
At this point the scene switches to Northworld itself, where Hansen's arrival is expected by the expedition members. They have discovered a strange energy field called the Matrix, which when entered (a difficult and dangerous move) gives them immortality and apparently limitless power – in effect, they have become gods (not really a spoiler – this all comes out in the first 30 or so pages and is just the background to the action). The Matrix consists of eight coterminous worlds, separate planes of existence, each of which has its own geography and climate – and life. The gods can instantly move from one to the other at will. There are also two bubble universes created by the gods, called Diamond and Ruby – the first being entirely peaceful, balanced by Ruby which is organised for perpetual warfare.
Hansen duly arrives and, after an initial visit to Diamond, he finds himself on Northworld (all of this being managed by the gods, led by North, who has plans for Hansen). The culture Hansen discovers is strange, to put it mildly: it is essentially based on the world and society of the Icelandic sagas (in which the author is clearly an authority), with small settlements constantly at war with each other. The difference is that the fighters wear battlesuits – suits of armour, powered by the Matrix, each with built-in electrical defences and weapons, advanced sensors and an AI to manage the systems. Much of the rest of the first volume alternates between Hansen using his battle skills to work his way up in the society, and the various machinations of the gods.
The description of the other two volumes necessarily involves a major spoiler for the ending of the first one – you have been warned!
Vengeance, the second volume, is a sequel but does not follow straight on from the first – some fifty years have passed, with the gods – who now include Hansen – remaining unchanged. New elements are introduced, including "smiths" who have a limited form of access to the Matrix, enabling them to create magical objects. Meanwhile, Hansen has a different task to undertake, although still in the same cultural plane of Northworld.
Justice is the final volume – and we have moved on another century. Hansen and North are now opposed in their desires for the future of Northworld, and find themselves fighting on opposite sides. There are also threats to the Matrix as the nature of Northworld becomes clearer, so Hansen must in effect be in two places at once – alternating between different planes in order to keep his various plates spinning. The ending explains the secret of Northworld, and how the humans become gods.
Northworld is an unusual and intriguing story, a blend of fantasy and SF which is a cut above the usual militaristic SF. There is a lot about battle tactics and brutal hand-to-hand fighting which not everyone will enjoy, but is probably quite realistic in its depiction of the world of the Icelandic sagas (which Drake also plundered for the plot lines, as he explains in more detail the author's notes at the end of each volume). I can't say that I am a fan of plots which include long time gaps – the longer the gaps, the greater the feeling of disconnect, and 100 years is rather long – but the author uses this to show the development of society as a result of the machinations of the gods. Overall, I think that this is an original and ambitious story which is worth reading, unless the military aspect turns you off.