Right now, I am 3/4 through Ring of Fire II (v. 2), edited by Eric Flint, who has apparently found a way to do without sleep. The Ring of Fire books, also known as the 1632 Series, belong to that small genre of "a large group of contemporary Americans find themselves sometime in the past for no discernible reason and proceed to change the world."
For example, S. M. Stirling has for his group the entire island of Nantucket. (And never a limerick anywhere in the books!) In the first volume, Island in the Sea of Timehe slams them into 1250 BCE, and if you have trouble picturing what the world was like then, they run into Odysseus. An interesting twist for Stirling is that he has two different series of books - one about the misplaced Nantucketeers and one about what happens on the world they left behind. (Given my druthers, I would have rather stayed with the Nantucket people. Just saying.) It's a sweeping world-wide story full of high fantasy tropes, engagingly improbable characters, dastardly villains and occasional lengthy and bloody battles. And one of the lead characters is a black, lesbian, martial arts master, Coast Guard Captain. (For which many people have condemned the books, saying that Stirling is PC and/or obscene, blah, blah. Whatever happened to SF people being the smart ones?) To say that his works are among my favorite for engrossing adventure and entertainment is an understatement. And his Draka books...woof. It's like the darkest of SM fantasies brought to life. When I reread those, I will go into more detail.
Flint, however, takes a small West Virginia mining town called Grantville and plops it into Europe in the middle of the 30 Years War. This series is impossibly huge, because Flint openly shares it with not only other professional writers, but with thousands of fans, who produce their own stories, some of which wind up in the supporting magazine and published anthologies and on the amazingly badly organized website for the series. I mean it, their website sucks rocks. For a bunch of people who get all geeky over the precise methods of manufacturing and warfare, they need a better web master.
That aside, I must say I enjoy the Stirling "Americans lost in time" more than the 1632 series, because of his masterful and epic writing style. (Plus, Flint looks like a High School teacher and Stirling looks like the guys I played D&D with.) But both the weakness and the strength of the 1632 series is the fact that it is a shared world. Sometimes, the novel or short story seems pedantic to the point of fetish. Other times, characters seem to change speech patterns and behavioral tics. But put together, there's this huge and varied world under construction here, spreading in all directions. So, an Italian action-adventure which has at its core a bunch of bumbling but well meaning teenagers off to rescue Galileo can act as a nice rest from reading way, way too much about various church doctrines of the time and their giant committee meetings. (At least for me.) Both the displaced Americans and the various locals appear as major and minor characters, and famous historical figures pop in for cameos or become major parts of the story. These books give me a good workout with Google; I am always looking up this king and that philosopher or artist. But then, I like that.
However, I do find myself skipping pages and sometimes entire stories of some of the contributors. It could be the topic, or the style. And there are events which unfurl over literally thousands of pages in the series; sometimes I need a codex to figure out in which volume this group gets rescued from the Tower of London, and in which book does this character die, etc., etc. A particular author can spend an entire story telling how various townspeople I can't really remember met and lived, while a central character who winds up being a sort of country-western version of James Bond seems to simply rise out of the text from nowhere.
And yet, when a new one comes out, I get it. And I enjoy...most of it. It's clear that Flint, et. al. want a more "reality based" version of the displaced Yankees story than the one Stirling tells. Stirling can't resist throwing some elements of fantasy and/or wishful thinking into the mix; but I appreciate that. But with a whole team of writers and fans constantly sending manuscripts, it's plain to see there will be simply *more* of the 1632 books. And among the quantity, there will always be some quality.