It was a collection of the works of Edgar Allen Poe, received it for my birthday when I was 8 years old.
Created a lifetime love of reading and a lifetime love of horror, fantasy, and SF. Led me to discover H.P. Lovecraft, Clark Ashton Smith, Ambrose Bierce, George MacDonald, Straub, King, Clarke, Asimov, Ellison, Vance and countless others. Side treks, specifically from Bierce, led me to Dickens, Wilkie Collins, Flaubert, the Bronte sisters and other lit that branched out from there. Reading was like a tree that grew from reading Poe, it was the catalyst for me for literary exploration.
Prior to that book I had only read fairy tales. Would I give my 8 year old child Poe? I would not. I'm still glad my mother gave it to me, one of the few things she did that actually impacted my life in a good way and gave me a lifetime love of reading.