If you are querying one book, and you have another MS on R&R with an up and coming publisher (but aren't necessarily looking for rep for that book), does it hurt or help your chances of getting representation to mention the R&R? Does an R&R with a publisher even matter to an agent when considering offering representation?
The only time you really need to mention that you have a manuscript with an editor or publisher is when an agent has requested material. Agents want to know who else has your work, even for a different manuscript.
Depending on the contract you sign, you could be tying up future manuscripts to that publisher, which will obviously affect whether an agent can sell the book you’re querying. It’s generally called Right of Refusal or First Right of Refusal, and it’s an important clause for every writer to evaluate before signing any contract. Publishers can possibly stick in there that they get first dibs on any manuscript you write in the future. That’s a big problem if you hope to get a book deal with a New York publisher.
So yes, mention it, but also be very attentive to everything in a contract before you sign. Some book deals are not worth the chance at publication. Writers who don’t know what they’re signing can easily get screwed over. Happens all the time, sadly.