According to George R. R. Martin's editor, Anne Groell, there may be eight  books in the A Song of Ice and Fire series instead of seven.

In response to some fan questions about her editing process, Anne Groell, long-time editor for George R. R. Martin, revealed that one of the few disagreements she has shared with Martin has been over the length of the series, which was originally intended to be a trilogy.  Though the pair agreed eventually that there would be “Seven Books for Seven Kingdoms,” Groell revealed that she believes there may end up being an eighth book.

Source: Vulture