It would be awful to have to pan a Hanff book. Her fans are so avid. Too, the chatty style of her memoirs, sweet and acerbic by turns, is so endearing that to criticize her too strenuously would be like senselessly striking a child. Fortunately, then, her latest book is good. Since the publication of 84, Charing Cross Road (1970), a collection of letters she exchanged with the personnel of a small British bookstore, her readers have wanted to know more about Hanff. Here she tells them, starting from the point when her one-year scholarship at Temple University in Philadephia petered out. Her parents couldn't afford to further educate her during the Depression, so she took a typing course. For literary sustenance, she turned to the public library. There she found the work of the Cambridge don, Sir Arthur Quiller-Couch, the ""Q"" of the title. From ""Q"" she learned to write. For many years after she came to New York she lived the hand-to-mouth existence of the struggling writer. Of course, all that changed with the publication of 84, Charing CrossRoad, which became a cult phenomenon. The rest of these memoirs, then, is divided between London, where her first book became a successful play and she became a celebrity, and New York, where she leads a tidy, interesting life as a writer. Hanff's charm is such that when she exults ""What fortune teller would ever have had the nerve to predict that the best years of my life would turn out to be my old age?,"" we exult right along with her. A very nice book, indeed.