So this is a sidebar novel -- very much off to the side, telling a separate story that just happens to be set in the same universe -- to that long series. It's also the expansion of the novella of the same name, which was a book of its own from
It's the early 24th century -- or perhaps the late 23rd; dates are thin on the ground here, perhaps in an attempt to make it harder for the Internet Nitpicking Squad to detect any possible discrepancies between The Empress of Mars and the main Company sequence -- and the increasingly placid and old-maidish Earth has been shipping off as many of its oddballs and malcontents as possible to Mars as a workforce for the British Arean Corporation. But the initial terraforming plans proved to be too ambitious, and the BAC has radically downsized.
And so one castoff from the BAC, biologist Mary "Mother" Griffith, decides to open Mars's first bar, staffing it at first with her three comely daughters and then increasingly with the quirky but surprisingly useful flotsam of Martian society -- and that bar gives The Empress of Mars its title. That's all backstory; this book is solidly in the SFnal tradition of bar tales in that evil forces are continually trying to shut down the Empress but her regulars and workers always manage to save it and take it to greater heights. (There are no stories told by people at the Empress in this book; in that one way, it does diverge from many similar bar stories of the past.)
Like the original novella, The Empress of Mars is the story of the triumph of pluck over adversity and of pints over prissiness. It's not one of Baker's most ambitious works, but it's an amusing story full of colorful characters, with a SFnal skin (that it does not do to think too closely about, in common with most of Baker's SF). If you've ever enjoyed lifting a pint, you'll find something to enjoy at The Empress of Mars.
Correction: The 2004 novella version of The Empress of Mars is from Night Shade; the 2009 limited hardcover of this version is from Subterranean. Screw future bibliographers; it's confusing me right now.