In the age of Google, you can’t hide. You can’t hide those old prom photos, your Twitter account, or your self-published book. And I’m not sure why you’d want to.
You self-published a book for a reason. Presumably, you were proud enough to want readers to find and read it. So why do I get so many questions about authors wondering if they should include that information in a query?
Of course, you should. You published it, now you need to market it to readers. You need to let people know it exists and you need to accept the idea that agents will find it anyway and wonder why you didn’t tell them.
Previously self-publishing a book, unless you have tens-of-thousands of readers will not likely help you find an agent. It will also not likely hurt your chances of finding an agent. It is what it is.
That being said, once a book is self-published finding an agent and publisher for that book is difficult. Extremely difficult. We don’t want to see a book you’ve already published. We want to see your next work.
Tell us you self-published a previous book and pitch the heck out of the next one.