In short, "Doc Sidhe" is about a down on his luck kickboxer that discovers a portal to the fairy world, an alternate universe that progressed to the cultural and technological level of the 1930s. The fairy world has human-sized fairies that live in cities with skyscrapers, but with a weirdly fairy tale edge: giant stone chimera and dragons adorn the art deco buildings. "Doc Sidhe" is a wizard/inventor from the fairy world, who befriends Harris.
If you want to make a checklist of steampunk elements this world has, you can take it almost beat by beat:
Airships. Check! I cut them some slack here because airships are cool, so I can understand the fascination.
Referring to things by old fashioned names, so you know it's an alternate universe? Check! One day, someone somewhere will visit an alternate universe where chocolate is called chocolate, but thus far, no dice. Spain is called "Castillan," Britain is "Cretanis." Like the Golden Compass, chocolate is called by a weird, Mesoamerican and historically accurate name, "xioc au lait," or "xioc with milk." Everything in fairy-ruled America is called by British names, like "lift" for elevators and "lorry" for trucks, and people drive on the left side of the road. This is to be expected in British-written fiction, but Anglophilia is one of the most bizarre American characteristics, and one that I unfortunately can't blame the South for, like I usually do with traits in my own country that I don't like (anti-intellectualism).
An aristocracy surviving to modern times? Check! Why do people find this exotic and interesting, anyway? The Star Wars or Dune-style resurrection of noble houses and monarchies in future or alternate societies always struck me as one of the more bizarre choices that writers make, since that particular stage in human development is behind us, even in places that historically had aristocracies like Sweden and Russia.
There are lots of cynical little details that make the world feel a lot less romantic, and thus more interesting, but frankly, not enough for my tastes. Darker-skinned fairies are unfortunately treated as shabbily in the fairy world as they were in the real 1930s for instance.
There are a few really great ideas in the story, among things that feel cliche for readers of industrial fantasy. The first is a superpower that, to the best of my knowledge, I have never seen anyone use before anywhere. The second is that, as a society becomes more complicated, "magic" becomes something that is technologically harnessed and studied. In the Fairy world, even the term magic itself is a discredited one.
What's more, the single most interesting thing about the plot is the main character, Harris, a down on his luck kickboxer. He's the classic textbook definition of the audience identification protagonist. He actually adds a lot to the story as a point of view character, something many pulp influenced stories are generally missing. When he goes on a mission with Doc's gang and has to plug a criminal, he feels horrible, something that a traditional hardboiled character wouldn't. His relationship with his girlfriend, who dumps him at the very beginning of the book, is the most interesting part of the story. Harris is a really nice guy, which is his problem: he lacks his own needs and wants of his own, and the reason his girlfriend left him is that she felt he wasn't a real person - just putty in the hands of anyone.
The character of Doc Sidhe only really becomes interesting at the very end of the novel, with a shocking twist that comes in the last 20 pages. Doc Savage was a great character, but like Tarzan, he's very distinctive, so anybody that is even slightly influenced by him shows his imprint. The way Aaron Allston created Doc Sidhe was to write him in a way that we think Doc Sidhe is identical to Savage...then, suddenly, have a sharp reversal that has all the more entertainment value because we don't see it coming. For instance, towards the end, we learn to our great surprise that Doc Sidhe has a girlfriend, a sexy Aztec fairy that is crazy, foreign, extroverted and says whatever comes to mind.
Aaron Allston is a supremely gifted writer at characterization. He creates people you like, and everyone had a personality trait, and you could never confuse dialogue from one character with dialogue from another. This book gives the feel of something written in a single draft, and believe it or not I mean that as a compliment: Allston was obviously figuring out who these characters are in the middle of the story, which is interesting to watch. It certainly means that unlike other books, it isn't something that starts off strong and then falls flat at the finish line; the book is "meh" in the middle and then quickly speeds up to a very, very strong finish.
Aaron Allston is extremely skilled at squeezing very powerful emotions. The death of one major character is much more profoundly moving than one would expect. One of the characters is Joseph, a golem-like monster made of clay created by a criminal that is a gentle being that in the past, was compelled to perform murders. Needless to say, his story does not have a happy ending.
In terms of action and adventure, Aaron Allston's characters aren't immortal and assured of victory. You get nervous for them because of their mortality, and how much you find you care about the characters.
In short, I'd recommend it.