It's the movie, plain and simple. The twists in The Maltese Falcon are largely lifted from the earlier Op stories - Bob Teal's murder opens the Falcon story and carries that first mystery all the way through to the end. The funny thing is, The Maltese Falcon is a remake of a film 10 years earlier that is nearly the same film, in fact Sydney Greenstreet and Peter Lorre are riffing off the first film's versions of their characters, it's eerie.
Oddly, I think Bogart would have been perfect as The Op, could have made a series out of that. He's short, can do overweight, and certainly not classically handsome. But it's not what Hollywood wanted, sadly. Because of that film and that role for Bogie, Sam Spade became a household name for a private eye despite having just 1 good story in Hammett's writings (there are 3 more shorts but they're not as good). It's really the Op who is the private detective that should have been on everyone's mind.
When I started reading the Op's short stories, they were pretty dry but it was clear where Raymond Chandler got his inspiration. Once I got to the longer short stories, as well as the 2 Op books, the Op becomes more of a rounded character and works more in the violence and gray areas and it's like reading Philip Marlowe with 30% less description of the furniture. Hammett really created an archetype with the Op.
I read The Girl with the Silver Eyes last night, great stuff, lots of twists without feeling cheap and I was gladly surprised to find about 2/3rds in that it is the second half of a 2-parter, the first part I had read the night before. The Op mows down his own informant with the client's car.