Cathleen Crowell Webb, whose recantation of a rape story led to freedom for her alleged attacker, is writing a book, titled "Forgive Me," that should reach bookstores in October, her publisher said yesterday.

"She will not get one dime from the writing of this book," said her attorney, John McLario of Menomonee Falls, Wis. "It's always been her intention to give everything . . . to Gary Dotson," the man she once accused.

Dotson's attorney, however, says he has nothing in writing from Webb and complained that her book will hurt the market for the "more scholarly" book Dotson intends to write. Webb's decision to recant her 1977 rape accusation led to Dotson's release from prison six years after his conviction. A spokeswoman for Fleming H. Revell Publishing Co. would not say how much Webb was being paid to write the 192-page book with Marie Chapian, a California psychologist.

Shortly after she came forward with her recantation last March, Webb said: "I wish I was wealthy, I wish I could give him six years, and I wish I had a million dollars I could give him to start his life over with, but I don't . . . I'd give him anything I have . . ."New Actor Named to Play Lennon

Mark McGann, a British actor who grew up three miles from the birthplace of slain Beatle John Lennon, was chosen yesterday to portray the rock legend in an NBC movie, the network announced.

McGann was hired less than three weeks after another British actor, Mark Lindsay, was hired for the role in the film about Lennon and his wife, Yoko Ono. Lindsay was fired when it became known that his real name is Mark Chapman, the same name as the man who killed Lennon Dec. 8, 1980.

Said McGann: "I was very, very sad for the bloke. I hadn't even thought about screen-testing for the show before I read about what happened to him."