An activist investor has proposed to take Barnes & Noble private in a deal that would value the bookseller at $650 million.

Sandell Asset Management’s surprise move, reported by the Wall Street Journal on Thursday, comes nearly four months after the hedge fund made an impassioned plea for the company to go private, arguing that public markets were not fully appreciating the bookseller.

The proposal would value Barnes & Noble at $650 million — or more than $9 a share — roughly 36 percent above Wednesday’s closing price. The total deal size would be $750 million, including Barnes & Noble’s debt load.

Barnes & Noble shares soared as much as 18 percent to $7.80 in Thursday trades after The Wall Street Journal first reported news of the transaction.

Shares were recently up 8 percent at $7.13 in mid-afternoon trades Thursday.

Barnes & Noble scoffed at Sandell’s proposal, saying it would need $500 million of debt financing — a prospect it said was “unlikely.” It would also require current shareholders — including Chairman Leonard Riggio — to roll there shares into the new private venture owned by Sandell.

“The company does not take Sandell’s proposal as bona fide in that Sandell is the beneficial owner of 1 million common Barnes & Noble shares worth approximately $7 million,” Barnes & Noble said.

“Mr. Riggio has no intention of rolling his shares into such a transaction, and the Company believes a debt financing of $500 million is highly unlikely,” Barnes and Noble said.

It is not the first time that Barnes & Noble shares were rocked by rumors of going private.

Just three months ago the stock soared on a report that Barnes & Noble retained Guggenheim Partners to help it explore a sale. The bookseller denied the report.

When asked Thursday if it was working with Barnes & Noble on a transaction, Guggenheim replied, “no comment.”

Reps from Sandell did not immediately respond to requests to comment.