Eric Flints Ring of fire series is good, they actually get better as you go along. Start with 1632.
Basic premise: The town of Grantville West Virginia gets dumped back in time and space into Thuringia germany during the 30 years war, the good old boys from the local coal mine then procede to remake europe to suit their ideals.
The anthologies with stories by other authors are particularly good, 'Curio & Relic' about a retired tunnel rat & C&R collector who ends up taking over the towns reloading work is excellent. It's the only story I've read with a realistic use of the .577 T-Rex in combat.
Flints "1812 The rivers of war" is shaping up to be a great series as well. Another what if, what if the Cherokee, instead of embarking on the trail of tears, had seen the writing on the wall and bugged out early, setting up their own republic on the other side of the mississippi and started making cannon?
John Ringos Ghost series. Basic premise: Mike Harmon, 'ghost' washed out SEAL & university student likes to stalk girls. While doig so he witnesses a kidnapping, jumps on the back of the perps vehicles, ends up in a major gun fight with the bad guys, but the girl gets moved, ended up in the wheelwell of a 727, ends up in the best gunfight I've ever read..... bugger it, I can't give you any spoilers, just buy the book. If you like it you'll buy the rest. It costs me $55.00 to buy them in hardback here in Australia and it's worth it not to wait for the paperback. *warning* There are some heavy sex scenes, including rape, bondage & S&M. Thankfully these are fairly self contained and I simply skip those chapters.
Ringos stand alone books, Through the Looking Glass & Princess of Wands are very accurate about their gun handling. TTLG is an aliens invade through a stargate that appears on earth. They get thrown back at great cost by a scratch militia. Then more gates start opening..... POW is about a hot christian MILF soccer mom (sorry but that's pretty much the way she is described) who believes in the jesus, no cussing, the sanctity of marriage,only saying nice things about people, that her husband is the head of the house hold and what he says goes. She also has a blackbelt and a H&K USP. Then real demons appear....
SM Stirlings Island series is good. His Dies The Fire series is even better.
In The Island In The Sea of Time, Nantucket is enveloped by a strange force field that sends it back in time, swapping positions in time with the Nantucket of 3000 BC. A coast guard square rigger, the Eagle is in port and provides a basis for exploration and trade with Mycean Greece. Unfortunately one of the coast guard officers is a psychpath with delusions of being the next Stalin and the technological base to carry it off......
In Dies The Fire, at the same time as Nantucket disappears, in our world the lights go out. Permanently. Electricity, gun powder, explosives and internal combustion engines cease to work. The great dieing begins as cities starve, with no way to move food or people other than on foot. In the bitterroot ranges a light plane crashes and the former ranger piloting it does what he can to save his passengers, gradually linking up with other survivors to form a band and restart civilisation again. Swords, cavalry, bows and arrows, norman castles and the roman phalanx return as the weapons of choice. Four books in the series and more to come, there are hints in book four that book five will hook up the two series.
Also by Stirling is Conquistador. If you've read anything by H Beam Piper, such as Gunpowder God, you will like this. It's the best alternate universe/cross time story I've read. It's a stand alone book, although I wish it was a series. The war is over. A former force recon marine is now a department of fish and game investigator on the trail of smugglers of protected species. In a raid on a warehouse he finds a live dodo...... It get's better from there. Blackhawk helicopters beside WWII mosquito fighter bombers. trecking across the west in a land where there really are wild indians. Real live Nazis. Incas who wear greatful dead T shirts while cutting out hearts... I've reread it several times.