From the title and the cover painting, you would expect this to be alternate history, but the cover and title are extremely misleading. It is, in fact, an anthology of old "classic" science fiction stories. Two of the three editors of this compilation--Drake, and Flint-- are two of the most popular authors at Baen books, and Jim Baen, of course, is the publisher. The stated purpose of the collection is to showcase works that "turned the world upside down" for the editors-- the science fiction stories that shaped and focussed their thinking at a young age. So the book gives you a window to see what the classic SF influences were on (at least some of) the works that Baen Books publishes. Each story comes with an introduction or afterward (or both), by one of the three editors, explaining why this story was selected, and how it "turned the world upside down" for them. The stories range from 1933 ("Shambleau") to 1967 ("The Last Command"). Some have been highly reprinted; others never before in book form.
With that said, the quality of the stories is amazingly erratic. Some of them are genuine SF classics. Some of them are feel-good stories, fun plots but not well written. A handful of the stories are simply awful: "Code Three," for example, by justly-forgotten author Rick Raphael, is bad in almost every possible way: unbelievable society, wooden characters, no noticible plot, laughable speculation. Even this, though, is in its way a useful reminder not to look at the past with gilded glasses-- it wasn't all wonderful, some of it was forgettable indeed.
Overall, a good addition to a library of old classics of sf.