Title: Detective Book Club [series]
Description: Published by Walter J Black, this is the July, 1949 Detective Book Club Mystery Omnibus featuring (1) Staircase 4 by Helen Reilly, (2) Dead Lion by John and Emery Bonett, and (3)The Dark River by Philip Clark.  

STAIRCASE 4 (from the Barnes & Noble website) Death strikes down a man on the eve of his wedding to a lovely girl. The verdict is suicide, but the girl is certain it is murder—certain because of a closing door. Inspector McKee wonders, too, and soon both he and the girl have their hands full trying to catch up with an ingenious murderer who leaves a corpse-dotted trail.

DEAD LION: (from Google Books): Cyprian Druse was a ruthless literary London critic whose viperous pen destroyed many a reputation. When he is found dead, the police rule it as an accidental death, but his nephew, Simon Crane, thinks otherwise. Crane and his friend, Professor Mandrake, set out to discover the real story behind his demise.

DARK RIVER: (From Google Books): One evening Janet and Murray Rossier have a bitter fight. She retires upstairs and suddenly hears a scream. When she gets downstairs, she finds her friend Marcella has discovered Murray's corpse in the study. It appears to be suicide, but Janet knows better and embarks on a dangerous investigation to determine the sinister circumstances that led to her husband's death.
Subjects: Detective and mystery stories