Flowers for Your Grave - S1-E1
Corrected entry: Halfway through the episode Castle tells Beckett that he got her "an advance copy," but in the opening scenes there is a stack of these available at his book party: these are always for sale and signature by the author, and at shortly afterwards he is actually signing these. Beckett would have known this since she showed up at the party to invite Castle to the station for questioning. (00:01:18 - 00:24:58)
Correction: She was focused on arresting Castle so it is a possibility that she didn't notice, or wasn't paying attention. She only focuses on what matters at the moment, so this makes sense.