Despite trawling through the internet I've not been able to track down the story title. I know that it involved Batman and Robin solving a murder where the victim had left a clue as he was dying. He was an amateur artist who worked in a travelling circus and was slain while working on a painting. Even as he was dying he managed to scrawl a cryptic clue onto the corner of the canvas: a minus sign ( -) followed by the letter Q.
I know that in a previous post I've written that I'll never give away the solution to a mystery but I have to if I'm going to illustrate how I became hooked on detective stories. Please take this as a spoiler alert and, if you're an aficionado of early Batman and Robin stories in particular, skip the following paragraph if needs be.
Anyway, of the 4 or 5 suspects one had the surname Dial. The Batman, being the world's greatest detective and able to tell that an ex-con thrown into the mix was only a red herring, knew that Dial was the murderer. The story was written long before key pad telephones and I certainly read it when we still were on a "party line" and had to spin a DIAL when making a call. The only letter of the alphabet missing from such a dial is the letter Q, hence "minus Q". This struck me as being unbelievably clever especially since I was only about 7 or 8 at the time. Looking back I should probably have been puzzled by the convoluted path that a dying man's thoughts could take but I was overawed by Batman's deductive skills.