As far as I'm concerned, it's Moore's Swamp Thing which had the biggest impact on comics creators at the time, not Watchmen, not V for Vendetta, and not the little seen Miracleman, brilliant as they all were. Moore's swamp Thing came out of the blue and transformed the conception of what could be done in a commercial comic published by a major publisher. Without Swamp Thing, Moore's impact would be considerably less as it gave him the clout and credibility to enable DC to publish Watchmen and reprint and complete V for Vendetta. This series is a major event in the history of contemporary comics, an impact which is unparalleled by anything that came before. The terrain of commercial comics was changed forever by it.
This volume contains eight issues which culminate the climax and conclusion of Moore's American Gothic series. The climax confounded many readers at the time, despite the constant (and guided by new character John Constantine) foreshadowing of it. Here the ongoing story is the thing with only three standalone issues, one a scathing attack on gun culture, another dealing with a serial killer which was later developed and amplified by Neil Gaiman in The Sandman.
Speaking of whom, he is one of two writers who contribute introductions to the book. And here is where my grouch wakes up and gets ugly. The presentation of this series is not all that it could be and that it deserves. The paper quality is poor, little better, if that, than the paper of the original comic. The introductions were written for earlier paperback reprints of this series back in the late 80's. Something new would have been nice. Also the 8 issues per volume seems a little meager to me.
But don't let that put you off. It is what it is and it's likely to be the definitive presentation of Swamp Thing for some time so get it while you can. Every comics fan should have this series.