There is an exam guide on the ArcGIS Desktop Associate 19-001 certification page- below you will find a link to that page and another to the exam guide.
Best of luck studying!
In addition to the links Megan provided, there is also an Esri Press book that was published in 2018, Esri ArcGIS Desktop Associate Certification Study Guide for 10.5, ISBN: 9781589485143. Although this is for an older version of the exam, most of the core concepts should be the same.
Good luck on your exam!
Because the book was published for last year's version of the exam, the content may be slightly different from the EADA 19-001 exam. I would recommend looking at the PDF in the second link that Megan provided, which goes over the topics that will come up in the exam. When you are studying from the book, make sure that you are able to answer all of the topics covered in the list. If there are any topics you find the book does not cover, you should be able to look up the information in the Esri documentation on the website.
Hope this helps!