It's just possible for readers to come to the previous book in Kage Baker's series without having read the others, but I wouldn't recommend it with this one. The Machine's Child starts in the middle and ends with a great many unresolved questions. In between, though, it's a hell of a ride.
Baker makes an attempt to summarize the earlier books, but it's a sign of how complex the series has gotten that the summary takes up six pages. It's presented in her usual breezy style "Alec promptly had a nervous breakdown. Goodness, wouldn't you, at this point?" deceptively breezy, because you don't realize how much information you're absorbing while you're reading it. Still, it doesn't cover everything, and I'd urge novice readers to start earlier in the series.
Okay, but say you're not a novice reader, and you want to know what the new book is like. And the answer is a lot like the others. Which is to say funny and fast-paced and horrifying and inventive, sometimes all at the same time. But this one's also frustrating: there's no indication of when the series will end, and when all, or at least most, of the questions will be answered.
When we last met Mendoza, the Company operative... No, wait I'd better explain what a Company operative is first. Mendoza works for Zeus, Inc., an organization that possesses the secret of time travel and is engaged in retrieving artifacts from the past. To do this they pluck children out of their various timelines, turn them into immortal cyborgs, and put them to work.
When we last met Mendoza she had turned against the Company. Cyborgs can't be killed, but the Company sent her back to 300,000 BCE, to a kind of torture chamber run by a sadistic immortal. Her lover, a man who contains the consciousness of her two earlier lovers...
Okay, wait. In the previous books Mendoza, as she lived through the centuries, fell in love with two men: Nicholas Harpole, a sixteenth-century scholar involved in the religious wars, and Edward Bell-Fairfax, a government operative for Queen Victoria. In a way I am not going to explain here, they turn out to be the same person, and the same as her current lover, Alec Checkerfield, and their personalities now inhabit the same brain.
Alec, and the other two, rescue Mendoza from her prison, and they continue their struggle against the Company. We see them traveling through the centuries, having adventures as they invest money and hide nanobot weapons, guided by Alec's AI Captain Morgan. (Captain Morgan is... no, I'm not going to explain that either.) And we see the beginnings of a conflict among Nicholas, the man of faith, Edward, the man of reason, and Alec, also a child of his era, who just wants to have a good time.
Watching these characters interact is a large part of the fun. Although the men share the same DNA, they still have many of the opinions and attitudes they grew up with. Nicholas has seen so many strange and horrifying things in the future he is losing his faith. Edward, who lived closer to Alec's time, fares a bit better. And in his previous life he has had to do some unpleasant things for queen and country, so the other increasingly rely on him in a crisis. But Edward, as it turns out, wants a body of his own.
Baker does a great job at differentiating them, a lot of it through dialog. Her biography says that she's taught Elizabethan English as a second language, and it shows; Nicholas's speech is not the mock-Tudor so prevalent in other books. Edward is an nineteenth-century English gentleman, and for Captain Morgan, every day is Talk Like a Pirate Day.
There are also glimpses into the lives of characters from the earlier novels. Joseph, the immortal who recruited Mendoza, continues to search for her; Suleyman is trying to find out what happened to all the immortals who disappeared; and Sarai, who was once Alec's nanny, is still looking for him. Clearly Baker is setting the scene here for the final battle against Zeus, Inc., and for an explanation of what happens after the Great Silence in 2355, a time when all communication from the Company ceases.
It's a bit disjointed, though, and the climax is more about the characters than Zeus, Inc., but it's all probably necessary for what's to come. Baker is a master at dropping in details that turn out to be important several books down the line; she did this even as far back as the first one, The Garden of Iden, which at the time seemed a straightforward adventure story. Still, it's disconcerting to come across a reference to William Randolph Hearst and then never hear about him again. (I'll go out on a limb with a prediction here: Hearst is going to be very important in the coming novel or novels.)
And, as I said, the book ends with a good deal unresolved. In fact, this isn't a book so much as a swath cut out from a larger story. I wouldn't feel so frustrated if I hadn't been caught up in the narrative, if I didn't want to know what happens next. The most pressing question at the end might well be "So when is the next one coming out?"