Over the last few months I've sent my novel to several publishers that accept unsolicited mss, and have received positive replies along the lines of 'Our readers enjoyed it, and we seriously considered publishing it, but it's not quite right for us.' One publisher got as far as requesting to see a second draft, then rejected it because I hadn't made all the changes they wanted. I'm now planning to approach agents in the hope of getting my work in front of other publishers. In my cover letter, should I tell the agent of the responses I've had so far?
You don't have to mention all of this in your cover letter, because ideally the agent should read the manuscript on its merits, not influenced by the thought that it's been rejected. However, you will absolutely need to tell them if they offer to represent you, because it affects their ability to place your manuscript. The agent won't be able to send the manuscript to the publishers who have already rejected it, and that restricts their ability to help you get published.