The stainless steel rat's revenge.

I liked the premise (invading a planet from another planet is impossible - but these guys are doing it - go figure out how). The infiltration and action were just as clever and fun as in the first book. The information gained late in the book seems to hint at a recurring villain. The only thing that lets this down is the solution to the original problem - it turns out they are successful invading planets because... they convert all the resources to invading the next planet? Not terribly convincing.