Parker was extremely prolific, producing over 50 novels, 37 of which featured the singularly-named Spenser, which also served as the inspiration for 1980s TV series Spenser For Hire (starring the late Robert Urich). Parker's first novel -- also his first Spenser -- was The Godwulf Manuscript, in 1973.
Parker had also developed three other regular series: Jesse Stone -- 9 books featuring a disgraced California cop who sets up shop as a small-town Police captain on the East Coast (starting in 1997) -- turned into a series of TV-movies starring Tom Selleck in the title role; Sunny Randall, 6 books featuring a female Boston P.I. (starting in 1999); and a western series featuring Virgil Cole and Everett Hitch, which had reached 3 volumes from 2005 to 2009. Parker was also a huge fan of Raymond Chandler, and wrote two follow-up volumes to Chandler's works, one (Poodle Springs) completing an unfinished Chandler manuscript, and the 2nd (Perchance to Dream) completely original.
Parker is considered groundbreaking for developing the "thinking man's private eye", and opened doors for many of today's popular hard-boiled authors.
Boston Herald obit for Robert B. Parker
Washington Post obit for Robert B. Parker
Wikipedia page for Robert Parker (with complete list of credits)
FantasticFiction page for Robert Parker (with complete list of credits)
Internet Movie Database page for Robert B. Parker (with list of screen credits)