Poul Anderson has long been one of my favourite mainstream writers. His first works were published in the 1940s, and he continued writing until his death in 2001. There's a huge body to choose from.

His primary strengths are two-fold. First is his skill and research at creating realistic alien worlds, with vegetation, climates, and especially names. Second is his Danish descent, which has meant that his works have always had a theme of sadness that is missing from the optimism of most American SF.

The Rebel Worlds is the third novel in the series of novels and stories about one Dominic Flandry, agent of the Terran Empire in the 31st Century. He's still young, a bit wet around the ears, and thus gets sent out to see what he can find out about a potential revolution at one end of the Empire whilst a large confrontation goes on with the Merseians (the other big empire) at almost the opposite end. Flandry flies around, takes action that is somewhat questionable given his authority, deals with the problem, and (barely) gets away with it.

This is not why we're interested in the novel, of course.

At one point Flandry crashes on Virgil 3, home of an alien sophont race just learning technology known as the Didonians -- as they call their home world Dido. It's the Didonians that make the book interesting.

They're not a single species. Instead they're three species, one something like a rhino, one something like a monkey, and one something like a bird, all three of which must link together in a symbiotic relationship to form a sophont, or sentient tool-using civilized individual. Where the transformation comes in, is that the same components can join with different components to create different entities. For example, one of the flyers could form one entity with one combination of rhino and monkey, and a completely different entity with a different rhino and monkey.

It's a fascinating idea, and, sadly, Poul Anderson only scratches the surface as the Didonians are almost a side trip in Flandry's journey. He does make use of one of them, using the fact that it is three creatures but one entity to his advantage, but that's about it.

So, consider this an introduction to an idea with fascinating possibilities. It's a short novel, fast paced, just enough to kill an afternoon -- and create months of imagination. Remember, a transformation does not have to be physical...