John Christopher's original "Tripods" trilogy has become a classic, and rightly so: A boy in a kind-of-medieval society discovers that the local rite of passage would end in him being a rather peaceful, unobstrusive, obedient and quiet conformer. And a follower of the Tripods, metallic giants everyone seems to take for granted.
In the course of the trilogy, the boy escapes, finds new friends and learns about the eventual plans of the Tripods - it's still a thrilling, great science fiction story especially for younger readers.
"When the Tripods came" is a prequel that sheds light on the original Tripod invasion, how and why they could win it, and other questions the original trilogy did not address. It's more a crossing of the t's and dotting of the i's than a must-read, and if you know the original trilogy, you will probably guess right what will happen most of the time. It's a bit like visiting a place where you grew up.
Still, the book contains some very interesting ideas, and makes a good travel companion (for a short trip or a long flight). The book is primarily aimed at teens; but even if you are a Tripods veteran who has long passed age 20 you will probably enjoy it - like in the original trilogy, the plot is coherent enough and the characters believable enough so that the whole story works just fine.
And if you have children of your own who you would like to introduce to a classic, this (sadly: rather short) book is a good starting point.