Michael Wolff
We have had such a great response to the Desktop Associate Study guide, that we have an author currently working on the Desktop Professional guide. However, this process takes about 6 months, and I assume you don't want to wait for that publication.
So, my recommendation would be to look through the qualifications and skills measured for both exams to see if there is significant overlap in subject matter. If you find that the topics from the Associate exam align with topics you want to study, then you will be able to make an informed decision. Additionally, I would recommend downloading the Preparation Resources document to review any training courses that might correlate to your topics of interest. Also, check out the free web course with sample questions for each exam that can help you prepare.
If you still have questions, please feel free to reach out to our team at certification@esri.com
JRosales _GIS Jessi Mielke