Thomas A. Edison
Multiples brass and bronze accessories.
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It's my opinion that your thoughts are on the right track. Steampunk takes a lot of its appeal from Art Deco and having some (fictional) functional things with flowing forms thats are easy to the eyes. I think your steampunk books would be a lot better if you designed them so that all the little things like gears, pipes, wires, gauges and whatnots would look like they serve a function. Arranging them like a lock mechanism, a restrainer (for evil books) or just doing some nice brass art deco elements seem like a good idea. Gears alone just look lonely/useless.
By the way: I really like the look of your non steampunk books. The design looks more consistent. They either represent something somebody would use in rituals, some evil spirits or creatures bound to a book or just expressing some affiliation.