I would either handle this according to the type of document (@booklet, @manual, @misc, @phdthesis, ...) and use a suitable field or just simply declare it as @unpublished in the BibTeX database (*.bib). Perhaps you can find more information (and inspiration) in the BibTeX manual.
Weird, now I put "year={preprint}", and it works! I get "(AUTHOR, preprint)". I remember it did not work before. Maybe it was before I switched to natbib...