Late last year I finished writing a "work for hire" manuscript of a travel guide to Spain, due to be published in the spring of '08. My questions are 1) at what point can I claim this as a pub. credit (i.e. do I have to wait until next year?) and 2) how do I reference this kind of work, as I have no rights as the author?
You can claim it now. You write "I completed the work for hire project Senorita Snark Slinks Through Seville (Publisher: forthcoming 2008)".
You mention it only if you don't have anything else to mention. As pub credits go, this isn't top drawer but at least it's something.
Agents understand your name won't be on the cover or copyright page. On the other hand don't be tempted to embellish. You don't know if I know the publisher or will check up on what you tell me.