It's sad to learn of his death. I don't think of him as a giant of literature, but he created an interesting character in Spenser, and I think the quality of his work was generally quite good. I enjoy mystery novels, and I enjoyed his work because it had much in common with that of Raymond Chandler, but managed to be different. His hero was tough (sometimes, rather fantastically so) and street-smart but also well read and even learned. Marlowe studied the games of chess masters (he seems to have had a preference for Steinmetz), Spenser quoted great poets, including my personal favorite, Wallace Stevens. But, unlike Philo Vance, Spenser didn't flaunt his intelligence or the fact that he was enormously well read although, unlike Marlowe, he didn't seem to mind letting people (even street types) know this about himself.
I think it's fair to say that at least as far as the Spenser novels are concerned, his work was getting progressively worse. The all-too-frequent exchanges between Spenser and Susan Silverman were beoming so coy, so cloying, as to be irritating. Spenser and Hawk were taking on the status of superhumans when compared with their increasingly miserable opponents, with the exception of The Grey Man, perhaps. He seemed to try rather too hard to populate his Justice League (Spenser's buddies who would sometime join him in battling evil) with representatives of most ethnic groups and sexual persuasions. It was somewhat creepy when the first Pearl the Wonder Dog died, and Spenser found a dog exactly like her and brought her home to Susan, who named her Pearl,. Nevertheless I read all the Spenser novels, and am sorry we will see no more. I never could get myself to read the Sunny Randall novels, but thought some of the Jesse Stone stories were good. I have a certain fondness for fictional characters who are alcoholics (and so have enjoyed Lawrence Block's stories as well).
I won't mourn the passing of Mr. Parker in quite the same way as I did that of Patrick O'Brian. I'll miss Jack Aubry and Stephen Maturin far more than I will Spenser. Still, there is one less good book to anticipate in life, and that is good cause for sadness.