The Pool of Fire, by John Christopher, was published in 1968 by Aladdin. It is the sequel to The City of Gold and Lead.
After being held captive in the city of Gold and Lead—the capital, where the creatures that control the mechanical, monstrous Tripods live—Will believes that he’s learned everything he needs to know to story them. He has discovered the source of their power, and with this new knowledge, Will and his friends plan to return to the City of Gold and Lead to take down the Masters once and for all. Although Will and his friends have planned everything down to the minute, the Masters still have surprises in store. Will enters the battle with confidence, but it might not be enough to fight against the Tripods. And with the Masters’ plan to destroy Earth completely, Will may have just started the war that will end it all.