Isaac, as the commenter says, did lots of good stuff, not just on aliens, but also on weird chemistries and strange physics. For a look at a how-to on coming up with aliens that make biological sense, They came from outer space: Real Aliens, published in 1997. The whole business of imagining aliens is a great topic. Until we've found a few thousand other civilizations and get hemmed in by reality, there's not much in the way of limits, either.
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Isaac, as the commenter says, did lots of good stuff, not just on aliens, but also on weird chemistries and strange physics. For a look at a how-to on coming up with aliens that make biological sense, They came from outer space: Real Aliens, published in 1997. The whole business of imagining aliens is a great topic. Until we've found a few thousand other civilizations and get hemmed in by reality, there's not much in the way of limits, either.