These books are basically works whose sales remain consistant over the years. One of the traits often used to define bestselling books is that they quickly go out of style and their sales plummet. Others, on the other hand, manage to stay at the top of the charts and are naturally the most valued by publishers.
While the initial momentum helps, the possibility of a bestseller becoming a long-seller multiplies if the work is of quality. The word of mouth among the reading community is another key factor. In addition, a book can only become a long-seller if it is never discontinued and always available in bookstores.