Even better, the publisher gives you the first book for freeHarry Potter and guns? That sounds like a potential Larry Correia fan to me!
Welcome to Monster Hunter International.
Five days after Owen Zastava Pitt pushed his insufferable boss out of a fourteenth story window, he woke up in the hospital with a scarred face, an unbelievable memory, and a job offer.
Note that the "insufferable boss" had turned out to be a werewolf. Anyway, Correia used to own a machine gun store, so he does get the gun stuff right.
I've found Donaldson hit or miss, and it tends to take him a novel to get going. That said, the first Covenant trilogy is amazingI've always liked Stephen R. Donaldson, anything by him. Thomas Covenant & GAP series won't steer you wrong if you like that sort of thing.
Ender's Game series by Orson Scott Card. Go Dragons.
Destroyermen by Taylor Anderson looks good too I started but never got around to finishing. Its about a WW II destroyer that gets sent to an alternate earth and becomes the most powerful ship in the world. Except for the Yamato, which also came along.
Speaking of alternate earths, there's always Harry Turtledove's Worldwar series. Alien invasion right in the middle of WW II. They plop right between the Russians and Germans, see that the natives (us) are fighting, and attack in both directions. All countries are forced to unite, sort of, to deal with the aliens.
Rama series by Arthur C. Clark.
Anything by Heinlein, as in Robert Heinlein. Starship Troopers, Puppet Masters, Childhood's End. All good stuff. If you read Starship Troopers, also read the Forever War for a contrast.