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Initials Only by American writer Anna Katharine Green is an early detective novel from 1911. (US Edition) (UK Edition) It is one of those books where I don't have to write a thing, but just quote from the wonderful Amazon Reader reviews:
I am half way through Initials Only, and its great - some of the passages of detective work are just flawless and first rate - better than anything written these days, (well, as good as the best)...it's locked me in and I loved it. Green really lays out the clues, and I am already looking forward to reading it a second time actually just to see the expertise in her work. Truly a great book. Enjoy!"