"Maus" was originally published in one-chapter installments in the comic anthology RAW (edited by Spiegelman and his wife). Later, when Volume 1 was published in 1987, critics found it extremely difficult to consign "Maus" to a literary genre. People have called it a memoir, a novel, a biography, a comic and a work of fiction (how could cats and mice talk, in reality?-- people asked). Eventually, critics and Spiegelman settled on "graphic novel."
In "Maus" itself and in many interviews after "Maus" was published, Spiegelman seems his biggest critic. When the author achieved literary success, he questioned whether he was ethical in using his father's story for his art. Further, although he felt that telling his father's (and other survivors') stories was necessary, he continually questioned whether his art was adequate to portray the horror of the Holocaust.
In Spiegelman's research, he found many Hitler-era propaganda publications depicting the Jews as rats or vermin; these images, along with his previous attempts to anthropomorphize humans in earlier comics, led to his artistic decisions in "Maus." Upon publication of Spiegelman's work, many people negatively reacted to its use of animals. Some critics found this use insensitive for Holocaust work, while others specifically criticized Spiegelman's depiction of the Poles as pigs.
A 25th anniversary volume of "Maus"-- "MetaMaus" -- includes digital copies of Volumes I and II, along with interviews with Spiegelman, his wife and his children about his creation of the graphic novels. It also includes archival video and audio of his interviews with his father, essays by contemporary comic scholars and critics, as well as never-before-seen drawings by Spiegelman.