I did read The Years of Rice and Salt a few months ago. (I'll read almost anything by KSR at this point.) I wasn't as fond of this book as I was of the Mars trilogy, but it's still fairly good.
The main thing the books have in common is, well, their humanity. He does a good job of developing characters in both works, and you really get a feeling for who the characters are, how they feel, and what they care about. The scope is also similarly sweeping in both. I'm not very good at comparing books, really, so I'll just throw out a few of my observations of the two.
TYoRaS stands way above most of the other alternative history or "historical SF" I've read. I'm not a huge fan of the genre, but I have read a few of Poul Anderson's books. Anderson's The Boat of a Million Years is in a similar league, but doesn't make you think as much, and his Time Patrol series is fluff compared to Robinson's book. (Entertaining fluff sometimes, but still fluff.) On the other hand, Connie Willis' Doomsday Book, which could arguably included in this genre, is much better than either TYoRaS or Anderson's stuff. It's one of the best books -- SF or otherwise -- I've ever read. Anyone who doesn't find themself caring about the characters in it is a robot.
I found the explorations of other cultures in TYoRaS very interesting, especially the parts about Islam. I'm not an expert on religions either, but the conflict between the different interpretations of Islam rang true to me. The aspect of the book I liked least was the whole reincarnation thing. I don't have any philosophical objection to it; I just found it confusing. "Now wait. Who is this character again?" Maybe some of it was because I have a short attention span. For whatever reason, it made it harder for me to care about the characters as much.
The Mars trilogy is also a strong exploration of human motives and emotions. Its characters "grabbed" me a lot more than in the other book. Some of this is because most of them are scientists and engineers, and I resonate with that mindset because I'm one too. The characters definitely aren't one-dimensional. Some of them change quite a bit through the series, especially if you read all three books. There's a joke near the end about a secret brain-reversal ray having been applied to a couple of them.
As I said before, the planet itself figures fairly strongly in the Mars books. Robinson does a very good job of setting scenes and making you feel like you're really there in Valles Marinaris, on the northern ice cap, or lost in a dust storm up on the plateaus. The technology also plays a fairly strong role, particularly some posited advances in automation and nanotechnology. They're within the realm of possibility, but I had to suspend my disbelief a bit. Some people might find these parts tedious, but I liked them.
There were a few parts of the books that bogged down a bit for me. The most notable was the exploration of the character Michel's (a psychiatrist) theories about personality, psychology, and philosophy. Some of it was interesting, but it seemed to drag on. A few of the "bad guys" seemed a bit one-dimensional too, though some of them do develop through the series.
On the whole, I'd definitely recommend the Mars books, particularly the first one.
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