INVASION! was DC’s 1988 crossover event and dealt with an Alien Alliance invading Earth in order to harvest its superheroes and, ultimately, make them the soldiers of the Alliance’s de facto leaders, the Dominators. Probably it’s most longstanding repercussion was the introduction of the meta-gene which was then used as an explanation of how and why some people gained superpowers.
The collected edition – re-titled as INVASION! SECRET NO MORE – contains the three main issues of the series and nothing more; not even a new introduction.
There’s a small collection of House Ads for the miniseries and some of its spin-offs/new titles here.
The pre-INVASION! Alliance
A couple of years after INVASION! a throwaway scene in GREEN LANTERN: EMERALD DAWN established the Alliance wasn’t as sudden as we’d thought. You can read about it here.
INVASION! on TV
The CW’s successful run of TV programmes based on DC characters had its third annual crossover in 2016 and chose to use INVASION! as the basis for the story. You can read the Wikipedia article on it here.