Harper Lee’s Go Set A Watchman is not the only recently discovered work to be breaking records this summer. Dr Seuss’s long-forgotten manuscript What Pet Should I Get? has sold 200,000 copies in just a week, its American publisher has announced.

The rhyming guide to choosing an animal companion has some way before it matches the 1.1m sales Lee notched up in the first week after publication, but, according to Random House Children’s Books, the latest addition to the Seuss canon is the fastest-selling picture book in its history.

The excitement around the publication of a previously unseen title from the beloved author of The Cat in the Hat even prompted a rhyming review in the New York Times from Michiko Kakutani: “Seuss never spoke down to his readers, no matter how small. / His tales were told with vim, vigor and zest. / What Pet Should I Get? entertains us just fine. / Who cares if this book’s not really his best?” she wrote.

A young reader gets stuck in to What Pet Should I Get? Photograph: Joe Raedle/Getty Images

Random House president and publisher Barbara Marcus said the “picture book treasure now sits prominently among the fastest-selling picture books of all time –and we couldn’t be prouder to share it with readers everywhere”.

Susan Brandt of Dr Seuss Enterprises said the author “was passionate about the idea that learning to read should be fun, and we know he would be overjoyed to see this book reaching and inspiring a new generation in just that way, yet again”.

What Pet Should I Get?, in which a brother and sister visit a pet shop but find it impossible to choose which animal to bring home, is described by Random House as “the literary equivalent of buried treasure”. After Seuss died in 1991, it was gathered along with other drafts into a box by his widow Audrey Geisel, and put aside until 2013 when the full text and illustrations were found again by Geisel and her assistant.

Theodore Geisel, who wrote as Dr Seuss, died in 1991. Photograph: Mark Kauffman/Time & Life Pictures/Getty Images

Random House believes it was written after The Cat in the Hat in 1957, and before 1962. In 1960, Seuss published One Fish, Two Fish, Red Fish, Blue Fish, which features identical children.

“While undeniably special, it is not surprising to me that we found this because Ted always worked on multiple projects and started new things all the time – he was constantly writing and drawing and coming up with ideas for new stories,” said Audrey Geisel last February.

Two more books are planned to be published from the materials discovered in 2013. As yet, no publication plans have been announced for the UK.