I have a query regarding resubmitting the same book to a literary agent. Once a book has been submitted it doesn't necessarily mean it stops changing, often it is revised and revised as we make improvements. Maybe the query letter wasn't up to scratch. Or the synopsis didn't reflect the scope and variety of your novel. Or the first few chapters were the first you wrote and required major revision that you couldn't see yourself at the time until someone pointed it out.
Is it wrong to resubmit your novel again to the same agent? If not how long should you wait? And should you mention it has been submitted before?
The term 'wrong' is a bit loaded ... 'Unwise' is probably better. You can certainly submit the novel again to the same agent, but it's likely that the agent will remember or be able to check and that they'll automatically reject you again. You won't ever necessarily know why they rejected you the first time - it's not always because of the query letter, or the story. Sometimes the agent is really not the right agent for the novel.
There are some manuscripts that I'll never take on no matter how well written they are and how good the query letter is - anything about a serial killer who attacks children, for example, or a book about snakes, because I'm never going to love snakes. I would simply not be the right agent for such a book, but there may well be an agent out there who is. Unfortunately we don't usually have time to personalise our rejection letters so we don't have the opportunity to be explicit about the reasons why we said 'no', which does mean that writers may think that they can submit the same project a second time and receive a different answer. When an author has asked to resubmit, though, I've never changed my mind. And if they don't tell me it's a resubmission I usually remember the author or the story anyway and remember why I said 'no' the first time. It's the same with publishers: I can't resubmit something to them if they've already said 'no', even if the author has made changes. Because the reasons why the publisher said 'no' to my client would be as hard to define as the reasons why I said 'yes'.
All of this points to the necessity of making sure your novel/manuscript and your query letter are as good as they can possibly before you start submitting anywhere. You need to believe and act as though you only have one chance. Because, most of the time, you do. You may, of course, be the exception and get another opportunity, and if you're set on attempting that I'd suggest that you mention the earlier submission in your letter and state very clearly why you believe your novel should be considered anew. The more time that has passed since the first submission, the more likely it is that the agent will believe that you've made actual changes.