I perused this crime novel sometime in the late 90's, most likely 1998. It was a used paperback for sale at the local library. Likely an undocumented donation to aid library funding. I only read a dozen pages or so randomly flipping through it while I was waiting for someone. But I recall the storyline specifically because I was already using online communication myself.
A serial killer is loose in a large city, possibly Chicago or New York City during the winter. He is responsible for at least one child abduction and murder. I don't recall if he killed children exclusively, but he leaves handwritten notes with each victim. His communications show a penchant for using emoticons (the use of punctuation and letters to create smiley, sad, angry faces).
The police are apparently baffled by this, not having experienced online communication in the pre-internet days. At some point a journalist gets word of this and becomes involved. As it turns out the notes' language and use of emoticons strikes a chord because she believes it is someone she has been texting online with in a chatroom.
I vaguely recall the cover showed a body on a rooftop half-buried in the snow with a sheet of paper pinned to it. The cover art might have been a crime-scene photograph with a note pinned to it in one corner. In both cases, I believe there was possibly a smiley-face on the note.
I suspect the book itself could have been written between 1985-1991, but the specific storyline would have been lost on even publishers until at least 1991 or 1992 (before AOL in early 90's there was CompuServe and GEnie, as well as localized BBS's)