[Teresa Edgerton]

The advice used to be not to confuse agents and editors by mentioning other books you were working on, but now it seems they are only interested in taking on those authors who they feel have a career in the making -- which is to say, more than one book. So it is a good idea to mention that you are working on or have completed another book, and to say a very few words about what the book is about.

But, yes, they do prefer consistency. Telling them that you are working on something quite different from the book queried or submitted is not going to help you sell that book. (Although if they like your writing, they may ask to see the other.)

Telling an agent that another agent liked your book but didn't take it ... that doesn't sound like it would help. Because while you are saying that someone else thought it was good, you are also saying that someone else thought the book would be difficult to sell to a publisher. So the negative cancels out the positive.