The Tripods Trilogy is a series of young adult science fiction books. It was written by a gentleman named Samuel Youd under the pen name of John Christopher in 1967 and 1968. There is a 'prequel' book that was written much later, in 1988, which is not part of the original trilogy and does not need to be read before reading the three main books.
The original books are, in order: The White Mountains, The City of Gold and Lead, and The Pool of Fire; the 'prequel' book is entitled When the Tripods Came. The setting is future earth. Humans (well, most of them) have been enslaved by an invading alien race known only as the Masters. The Masters ride in enormous legged warmachines caled Tripods - hence the name. For most humans, the Tripods are as close as they will come to seeing their Masters. Devices called Caps are implanted in humans' skulls ceremonially at a young age. Some are then taken to serve as slaves inside the domed Cities of the Masters, wearing breathers to survive the poisonous green atmosphere (probably chlorine; I recall it ate earth metal). Inside each of these Cities of Gold and Lead is a power source, a burning pit called The Pool of Fire which must be extinguished if any attack on the cities is to succeed. An international contingent of young men who have evaded being Capped, led by the Elder Julius from a safe retreat high in The White Mountains of the Alps, undertake to do just that...
In 1984, the BBC began broadcasting what would be two seasons of a television adaptation of the Tripods trilogy called simply "The Tripods." The first season roughly matched up to The White Mountains, and the second to The City of Gold and Lead. The third season, which would have adapted The Pool of Fire, was never produced for a variety of reasons including a shift away from sci-fi on the part of the BBC management.