A publisher/editor friend of mine recently told me self-publishing with Amazon is relatively easy, definitely doable but it takes work. I have published two nonfiction books and easily got a publisher, one within 5 days and the other in two weeks. I have published two fiction books which took months to find a publisher. I just finished another fiction book and am back in the trenches trying to find an agent or publisher.
Because your book is nonfiction, if you have not done so already, I'd tried writing to a publisher. For nonfiction books, generally, you do not need an agent. Look at the publishers whose books you read and use on your topic. There are several helps for writers to find publishers and agents. I like Query Tracker. The basic service is free and the premium is $25 a year. You can set up your search by your category of book publishers you are looking for. Good luck