Now &Then by Robert B. Parker Putnam, 2007, $25.95, 296 pages, ISBN 978-0-399-15441-6 This week, I stuck my arm w-a-a-y down in the old book bag, and just like Jack Horner, pulled out a plum: an unread Spenser novel by Robert B.Parker titled, Now & Then.      The story begins when Spenser, Boston’s most well-known private investigator accepts a case from a man trying to find out if his