Barnes & Noble cut its Nook sales forecast for this year and shocked investors by saying it was considering a sale of the electronic reader and tablet business, sending its shares down sharply. The bookseller has been banking on the Nook for growth, so news that holiday sales of the basic touchscreen e-reader were disappointing raised investors’ fears that Barnes & Noble was struggling to keep up with Amazon.com’s Kindle.  ”They’re going to have to raise capital for Nook if they want to stay viable,” said Morningstar analyst Pete Wahlstrom.