GURPS Alternate Earths is published by Steve Jackson Games (ISBN 1-55634-318-3) at a suggested retail price of $17.95 Written by Kenneth Hite, Craig Neumeier and Michael S. Schiffer and edited by Susan Pinsonneault.
This volume, evidently, is intended as a companion to GURPS Time Travel, which was published some time back (and it was originally planned for an earlier release, before the CCG money crunch slowed game production at SJG).
Alternate Earths, in this case, refers to the concept of parallel worlds. Six are presented within the volume, including divergence points, altered histories, and a number of plot hooks for crosstime campaigning.
The authors tried hard, in the mere 128 pages they had to work with, to give decent presentations of the kinds of differences one would encounter in an alternative timeline, and how they might come about. Overall they did a much better job then, say, the tv show Sliders (on Fox). They don't fall into that sort of bad stereotyping, the choices for points of divergence are much better done, and the resulting differences are much less heavy-handed than the worlds on Sliders.
But trying to squish a history, cultural difference, and lots of detail into less than 30 pages per alternative world is a task I certainly would have found daunting (if not impossible).
Dixie, the first Earth, is a "What if the confederacy won?" timeline, taking it a few steps further through time and looking into the impact such an event would have by modern times, both in North America and beyond. The timeline, and resulting political situation, do not seem to follow historical logic, and so lack believablity.
The Reich is a plotline used far too frequently in comics and tv, the "What if Hitler had won?" concept. Far too many WWII stereotypes, and an extremely heavy, almost oppressive, concentration on the problems of an endless guerilla war against the nazis, fills this section of the book.
Roma Aeternal, the Roman Empire triumphant, is not fully brought up to modern times, but instead explores the new concept of "Steam Centurians". That is, the timeline has reached about 1878 and the Roman Empire (in this case the third one) has extended over most of the world. This is a little better than The Reich in concept and detail, and certainly more original. It would have been a good idea if they had cross referenced this with "GURPS ROME" (and might have been a good springboard to reissue that classic GURPS Sourcebook), and it certainly should have been given more space in the book (Perhaps the pages I feel were wasted on The Reich).
Shikaku-Mon is the most original timeline. I can't do the history justice here in only a paragraph, but I'll give it a try. Basically, the divergence occurs in the 16th century, when the heir to Ferdinand and Isabella of Spain survives to ascend the throne as John III. Without the union of Spain and Austria under the Hapsburgs, European history took a different shape during the reformation. Meanwhile, on the other side of the globe, the Jesuits were successful with their campaign to bring christain spiritualism to Japan. Under the oda chancellors, Japan began an early westernization that led to its dominance in the far east. The game setting is the 20th century, with the Japanese in control of the eastern half of the globe. Brazil is a major economic power. The Swedish Empire is a totalitarian power and the Kingdom of France has become the major power in the high ground of outer space. However, with so many changes the outline given barely scratches the surface of the resulting world. Again, this world is one that should be expanded, and in my opinion might have been made into a sourcebook all by itself.
Ezcalli is a world dominated by the Tenocha Empire, a heavy updating of the Aztec Empire. It's set in 1840, making it be a good alternative earth for steam punk time travel games. Although the ideas here are less used then some, the world is far too "thin" in content. Its not as original as Shikaku-Mon, and much of the information included is uninspiring, while the things which would titillate the curiosity are left out.
Gernsback is the last of the worlds. It is built from a timeline where science and technology have taken a left turn, making a future that looks like 1930s SF stories. The divergence is more particular and inventive (and for some of us quite amusing) : Nikola Tesla marrys the daughter of J.P. Morgan, linking the last of the popular mad scientists with the last robber baron's money. Instead of vanishing into bankrupt obscurity, Tesla took technology into a new path and with it changed global politics. The depression never occured, Hitler never rose to power and Germany, instead of Japan, is the major economic competitor with the USA. In 1965 the world sits in a state of technological utopia, prosperity, and glorious order (or so it seems). The technology is a wonderful hodge-podge, leaping ahead in some areas, while stagnating in others, so that some things we take for granted are non-existant. The history is logical (or at least as logical as Tesla ever gets) and the result is dynamic, with a real sense of wonder. But again, this world suffers from a lack of space (in particular, except for a few specific inventions, the effects on technology are barely explored) and could easily have been the basis for an entire worldbook.
In general I give the book a C+. It has some good ideas, and the work of some remarkably creative people, as well as some pieces of schlock. Steve Jackson Games didn't give the writers enough space for the best parts, and it shows. Perhaps condensing, using the three best of the world base concepts, would have give enough space to present the needed additional information. Unfortunately, it looks like the book was a victim of tightening game production budgets, and may be the harbringer of future work in the field.