Crossroad Press is offering this collection of Bill Pronzini short stories.
Pronzini has a huge list of published books, mostly hard-boiled detective stories, with the most famous being his Nameless Detective series. Similar feel to Lawrence Block, the Robert B. Parker 'Spenser'' series, etc.
A quick look shows that Crossroad Press features several other popular authors in a similar vein. Although I didn't spot any other free ones, I was pleased to see that a lot of their titles are very reasonably priced for ebooks.