Not actual books, but these are what I imagined a Twelve trilogy would resemble.
"Twelve - Genesis"
On the blue jewel of Nandonia, resides a humble race of people known only as the Snevele. For centuries, they have been driven to slavery and ruin by a cunning and intelligent group of space-sorcerers known as the Juusan. So wicked are their devices, so black are their plots and cruel their fetishes - that the Snevele have finally resorted to sacrificing their first born to the heavens. Praying and weeping that above the fury of thunder and hail, there would be salvation at last. Above the chaos of the cosmos, their pleas reach the most unlikely of pantheons. For he who received their cries, did so in a fury of mead, carnal intoxication and skulls. Their suffering and plight did give him pause, and compelled by his heart and loins - spurred him to aid these slovenly people from their malicious existence.
Upon a great and golden galactic fork, the receiver did ride. In a fury of lightning and steel, the great vehicle crashed and penetrated the planet. Nandonia's continents shook in an aching moan that resonated across the entire star system. In an instant, the Snevele got their wish - a cosmic lord that would hear their cries. As the wave of debris settled at the great thundered crater, a rustic figure shimmering in static emerges. However, his was not a crusade of salvation, redemption nor justice. There would be no relenting to the trickery and glamour of the Juusan captors, nor their Snelevan slaves. In these chronicles, journey with the Viking Lord Fork as he purges the filth from the blue jewel - and meets steel with older primal forces that have taken root on this lush world.
Fork is here. Hide or suffer, surrender or perish, kneel or die. Fork is here.
"Twelve - Whiskey Tango"
Nandonia in turmoil, bloodshed and fire as far as the eye can see. This fledgling world is at the brink of total implosion, as a war between the aggressive sea-borne Trollen empire and the continental superpowers of Moderatia erupts. What was once one in the same, has splintered into two heads of the Hydra - snapping in destructive lunges. Millions of lives are affected, and millions more are at threat. The blue jewel is now a sky of gray, as entire cities are bombarded and shelled to submission. No more pleas may reach the heavens - for the crack of the rifle, and the din of the mortar have all but drowned out the reason of men. With the age of the sky gods now long gone, the time for a reprieve is direly needed.
The cosmos does not answer the call. The last modicum of hope lies with a neutral combat specialist with ties to the Bowl of Spoons - an ancient valley of constructs rumoured to have existed long before the great thunder came to strike the world. Thought to be the source of power to ancient star-wizards, this monolith lies in the center of the hotly contested dreadlands to the south. Guarding the ancient monolith, lies the impregnable Fortress Gilgamesh - unbroken and undisturbed by the great inferno of conflict around it. They call him the Cricket, an unassuming specialist of infiltration and marksmanship, and leader of the now infamous freelancing Twelfth Squad. His mission - suicidal and fraught with the stench of death. His purpose - to seize the fabled Robe of Red, an artifact of Godly persuasion. In this story, deploy with Specialist Jiminy and his team, as they race to uncover the puzzles of the Bowl, and unlock this young planet from the final waltz with oblivion.
This is Cricket One to Control. Tell my family I am awesome. Jiminy out.
What it would look like on your book ends:
"Twelve - Valkyria"
Aboard the cosmic battlecruiser, "Hunterbob", a determined figure leaves behind a browning world. Part of a cadre of survivors from the unification wars, he embodies the journey of a people that has seen unending trials and tribulations over the centuries. The Hunter is finally on the prowl, with the clock ticking down as the planet's magnetic core weakens - threatening to undo the Great Fork of the north and the Bowl of Spoons in the south. Only those who gave power and life to the newborn world can hope to aid the crew of the Hunterbob and her sister ships in their quest. Only the great sky gods of the mythical Valhalla. He knows that in order to reach the gates of heaven, he must first cross the hellish crossroads of the great blackness.
There are forces in this galaxy that have stirred into life, forces that are unkind to interlopers in their domain of blackness. They too seek the sky gods, but for completely different reasons. They have mobilized their war machines and steeled themselves with a supernatural resolve. They will stop at nothing to ensure that the last lights of Nandonia are extinguished, and to ensure that none but them - hold the rights to commune with the great Asgardian lords of the Cosmos. In this final installation of the Twelveborn chronicles, crash through the continuum of spacial limitations with Captain Thorment and his crew aboard the Hunterbob. As the curtain closes on an aging world in need of life, twelve titanic warships break through the slipspace of uncharted stars. They will return victorious holding their shields, or drift in ruin for an eternity.
I'm not just calling the bet, Navigator - I'm upping the stakes. I want their shipmaster's epitaph to read, "I bet against Captain Thorment".