At 7:00 that night, Marlowe heads over to Puss Walgreen's office to see Jones. He hears talking coming from the office, and listens quietly from the door.
Eavesdropping's a necessary skill when you're a detective.
Jones is being interrogated by Canino, who says Mars wants to know why he's been tailing Marlowe. Jones tells Canino that he's blackmailing Marlowe for money for Agnes' drug habit because he has information on Carmen's involvement in Brody's murder. Jones tries to convince Canino that his interactions with Marlowe have nothing to do with Mars.
Canino asks Jones for Agnes' address, and Jones eventually gives him the info. Before Canino leaves, he offers Jones a drink, but it's poisoned with cyanide.
Marlowe waits until Canino is gone, and finds Jones' dead body on the office floor.
Marlowe finds the phone book to confirm Agnes' address. It turns out that there is no Agnes at the address that Jones gave Canino. Marlowe admires Jones for not ratting out Agnes.
A few seconds later, the phone rings. It's Agnes. Marlowe tells Agnes that Canino came by and Jones got scared and ran. They set up a meeting to exchange money for the information on Mona Grant.