I can think of a lot of things that Big Sleep is about. But I see what you mean. I've never thought that plot could be a bad thing, but in Chandler's books the characters, the setting, even the language had more significance than the plot itself, and that's probably why I relished every sentence of them. Too plot-driven and you just want to see what happens in the end. Never saw it that way before, but it makes sense!
Pity about the Black Mask Boys, I may try the paper books if I come across them.