I think it found it by perusing a forum thread asking for recommendations of books/stories about Modern Soldiers in Fantasy Settings.
Appears to be the DemonTech Series by David Sherman.
The Evil guys do a summoning that gives them tech manuals for weapons
and tactics I believe.
The good guys try to imitate it, but the demons which they use are
confused so they instead send the good guys a Marine Gunnery Sgt. He
goes on to train their Marines. Lord Gunny indeed.
There are four books:
The diabolic armies appear invincible– but they haven’t met the
The Dark Prince’s power to summon demons has made his forces second to
none, and ensured his place as one of the invasion’s leaders. Not
content to merely seize his father’s throne, the renegade royal dreams
of world conquest–and with his army and his black arts, there is
little to stop him.
So unexpected, so well planned is the invasion of the free port New
Bally that out of hundreds in the city, only two escape capture. Haft
and Spinner will need all their courage and cunning to retrieve their
weapons from their enemy-held ship, cross the open meadows surrounding
the city walls, and escape into the forest beyond–where they hope to
regroup and drive back the invaders. Luckily, Haft and Spinner are no
ordinary men. They are Marines . . .
Rally Point (2003)
Ruthless and seemingly indestructible, the dark army didn’t count on
the few, the proud, the Marines.
The Dark Prince’s vast armies have destroyed nation after nation with
the aid of powerful demon magic. Their total victory would be swift
and certain except for two things: Haft and Spinner, a pair of
soldiers trained in the art of warfare through the teachings of a
magically transported Marine Gunnery Sergeant.
The Dark Prince’s orders are swift and simple: find them. Haft and
Spinner, along with scores of soldiers and refugees, are determined to
form an army and defeat the invaders. Outnumbered and outgunned, they
must uncover and kill the enemy before the enemy kills them. Haft and
Spinner have already accomplished great feats. But to achieve the
impossible will require a few strokes of genius and a few good men.
Fortunately, these Marines have both. . . .
Gulf Run (2003)
Only a miracle can save them now. Luckily, they have one: the
Like an invincible tidal wave of evil, the Dark Prince’s conquering
armies roar across the land, leaving death and destruction in their
wake. So far Marines Haft and Spinner have managed to stay one step
ahead of the invaders, picking up hundreds of refugees and soldiers as
they struggle to reach their salvation at the edge of the sea. Against
all odds, they’ve even managed to inflict some damage on the vastly
Yet for every enemy soldier killed, ten more appear in his place, and
the Dark Prince’s demon magic ensures their weapons are brutally
effective. The outlook is not promising for the ragged band of rebels,
pursued by bloodthirsty troops, hounded by bandits, and forced across
a desert where death awaits all trespassers. As the noose tightens and
the Dark Prince prepares the final coup de grace, it may look like
Haft and Spinner and their embattled crew are finished. But these
Marines have only just begun to fight.
Get Her Back! (2010)
Send in the Marines! Because the Golden Girl is on the loose...
And it's up to Haft and the Bloody Axes to...Get Her Back!
When headstrong Alyine sets off on her own to find her left-behind
musician, life gets interesting for those who must go after her. Angry
nomads, great hunting cats, and desert death-matches stand between the
Golden Girl and her rescuers. Using every trick at their disposal,
Haft and his men are more than up to the challenge!
Have no doubt, they'll get their Girl!