2000  STEWART, I and COHEN, J.  Wheelers.  London: Simon and Schuster.  

A novel of first contact – but with Jovians; lots of well-considered alien biology, and some astronomical puzzles are worked out.

2003 COHEN, J. and STEWART, I  Heaven. Warner-Aspect, New York

Two thousand years after Wheelers, the religion has taken off – and fossilised; it must be resisted, at all costs.  Many well-designed, credible alien species (as well as Neanderthals) interact in an exciting story whose philosophy is ultimately surprising.