It Is Balloon !

The Pool of Fire is the third book in the Tripods trilogy.  In it the humans learn what Will Parker discovered in the previous book.  The aliens are sending a ship from their home planet to Earth with a machine for converting the atmosphere on Earth.  It will become like the home planet of the Masters, and all other life on Earth will die.
So, a daring plan to assault the three Tripod cities is devised. It involves poisoning the aliens.  It is successful in killing the Masters in two, but the attack fails in the third.
This leads to a further attack via a balloon born force. 
This book is much more action oriented, and perhaps a bit predictable, but still a good wrap-up of the trilogy. 
All in all a good trilogy of juvenile SF.  I will cover the prequel in my next post.