INCLUDES AN EXCLUSIVE INTERVIEW WITH THE READERS 'Your ad in the Saturday Review of Literature says that you specialize in out-of-print books. The phrase 'antiquarian book-sellers' scares me somewhat, as I equate 'antique' with expensive. I am a poor writer with an antiquarian taste in books and all the things I want are impossible to get over here except in very expensive rare editions, or in Barnes & Noble's grimy, marked-up schoolboy copies.' So begins the delightfully reticent love affair between Miss Helene Hanff of New York and Messrs Marks and Co, sellers of rare and secondhand books, at 84 Charing Cross Road, London. For 20 years, this outspoken New York writer and Frank Doel, a rather more restrained London bookseller carry on an increasingly touching correspondence. No doubt their letters would have continued, but in 1969, a letter informed Helene that Frank Doel had died. In the collection's penultimate entry, Helene Hanff urges a tourist friend, ''If you happen to pass by 84 Charing Cross Road, kiss it for me. I owe it so much.'