I find myself without an agent after the Very Impressive Agent and I decided not to go forward. He pronounced my latest piece very good but not what he sells.
I don't cry. I query. So I'm already out with the first queries for that novel he doesn't like, as well as separate queries to different agents for the one I shelved when he signed me for my first. (Sorry, but you will be hearing from me -- do you prefer the new spin on romantic suspense or the historical mystery first?)
He didn't sell my first book. We parted amicably. I plan to say very nice things about him and hope he will do the same. What, if anything, am I obligated to tell the next agent who hopefully will be a better fit?
If you're querying for the first book that he didn't sell, you have to reveal all, and up front. You had a previous agent, the book went on sub, it didn't sell.
I can assure you that this is NOT the book you want to query on. I wouldn't read past the "it went on sub" part in a query before saying no.
You want to query for a Shiny New Book that hasn't gone out. When you get bites, you can talk about your inventory (which will include the book that went out on sub.)
When querying Shiny New Book, you don't need to mention the previous agent. You should mention it in any phone call with agents considering representation. You should be prepared to discuss what didn't work with that agent, and what you're looking for in a new agent.
You're better situated to know what a good working relationship with an agent will look like now, having had one to learn from.
It's important to know what works or doesn't cause you really don't want to be parting from Agent Deux anytime soon.