Maus is available in several forms, but the two commonest are the two graphic novels Maus: A Survivor's Tale I: My Father Bleeds History and Maus: A Survivor's Tale II: And Here My Troubles Began or the Omnibus Maus that includes both the two previous volumes. I prefer the two separate volumes in part because it's how Spiegelman originally published them five years apart (and the second book makes it clear there's a lot of history behind that five year gap), but also because the omnibus volume is physically smaller, so the artwork wouldn't present as well.
I first read Maus years ago, but just purchased my own copy and re-read it. It's one of the most acclaimed pieces of Holocaust survivor literature, and was the first graphic novel to win the Pulitzer prize. And according to Wikipedia, it was (presumably partly because of the Pulitzer) the graphic novel that finally dragged comics out of the teenage ghetto and into the category of "literature."
On the surface, Spiegelman's choice (which he discusses in the book) to portray Jews as mice, Poles as pigs, and Germans as cats seems unusual, at odds with portraying such a horrific tale. But it works. Man does it work. Spiegelman's parents survived the Holocaust, and in 1978 he started interviewing his father Vladek (his mother died in 1968) about his experiences before and during the war. Maus is both the story of Vladek and his family's experiences during the war and also the frame story of Spiegelman's interactions with his father around the interviews.
The books are both spectacularly good and breath-takingly horrific. Vladek is a mess, but after you read this you really begin to understand why - and why horrors like this pass through generations and even leave the children of Holocaust survivors a mess themselves. A pair of graphic novels like this would normally take me a day to read, but I kept having to set this aside because it was so disturbing - I think it was four days before I finally got through it.
It should be required reading for the entire world.