you can list the book under works/books in progress, especially since you haven't secured a contract. I'm not sure if I agree with the comment that it has to be in print to be listed under publications, which seems to be overstating the case. If the (first book manuscript has been delivered to a publisher and has entered into the copyediting phase, it is perfectly acceptable to list it under publications as forthcoming ( I have never heard of case where the editors rejected an ms. at this stage) . If you have published more than one book (or multiple books as I been fortunate enough to have done) and have a good relationship with the editor/publisher, then a book under contract can be listed under publications. But this latter arrangement is of course a judgment call, one based on the strength and trust you have built up with this publisher and how far along the ms is. But I would add that this fear of overselling oneself can also lead to an underselling of oneself, which may be far worse a condition in today's market.
Reread: The OP not only has not secured a contract, but the OP has not even submitted the manuscript.
So, the rest of the above also is not relevant.
And so: In the cover letter, not on the c.v. -- and OP, take this contact from the publisher as incentive to move along fast on readying that manuscript for submission. Staff can change often at publishers, and that contact can be gone, and nobody else may know about this indication of interest. Ask me how I know. . . .