It was only supposed to be a short character sketch — but Yale professor emeritus Edmund Morgan went a little too far.

As chair of the administrative board that oversees Yale’s ongoing project, The Benjamin Franklin Papers, Morgan planned to write a guide for a CD-Rom containing the inventor’s collected letters.

“The character sketch kind of got out of hand and I couldn’t stop writing about it,” Morgan said. “This book is kind of an expanded character sketch.”

The final product was “Benjamin Franklin,” a biography that has received critical praise and has become Morgan’s most commercially successful book to date. The book made a brief appearance on the New York Times bestseller list and was announced as a finalist for the National Book Critics Circle Award in biography this week.

“It has been named a Best Book of 2002 by Publisher’s Weekly,, the L.A. Times,” said Brenda King, Morgan’s publicist.

Morgan, the author of numerous books, including “American Slavery, American Freedom” and “The Puritan Dilemma,” said he was surprised by the attention his latest effort has received. But Lara Heimert, Morgan’s executive editor, said that she had high hopes for the book from the beginning.

“Ed Morgan is such an established and beloved historian,” Heimert said. “We knew it would do well. I don’t think we knew how well it would do.”

Heimert said the book has broken records for the Yale University Press as well.

“I think this is the first time that a Yale University Press book has hit the New York Times bestseller list in hardback,” Heimert said.

Morgan, who turned 87 Thursday and taught history at Yale for 31 years, attributes the biography’s success to two factors.

“One is the geezer factor and that is that geezers aren’t expected to write books,” Morgan said. “The other is that it’s about Benjamin Franklin and that gives sort of automatic interest.”

Morgan said the book held special interest for him because it was a topic of his choosing, rather than one picked by a publisher. He was also in a unique position to write such a personal biography: because of his responsibilities overseeing the publication of Franklin’s papers, Morgan has read nearly everything that was ever written by or to Franklin.

Morgan said he is unsure if he is ready to begin a new book at his age.

“It usually takes me from six to 10 years to write a book,” Morgan said. “It might be a little presumptuous to start another.”

Nevertheless, Morgan said he was not sure that this book would be his last.

“I like to write,” Morgan said. “If you are a writer, you can’t stop.”

Heimert said she hoped that “Benjamin Franklin” was not the final Morgan book to hit the shelves.

“We would be thrilled to publish anything Ed Morgan writes,” Heimert said.