Carefully review the exam prep guide here:
https://www.redhat.com/training/rhce/examprep.html
To answer your question, I don't know of a current book / guide. I purchased the Red Hat Certified Engineer Linux Study Guide (author: Jang), and it was a major version behind the RHCE exam I sat for. It still helped, though.
One more thing: If you're not a GNU/Linux wizard who knows RHEL inside and out, and if you can't confidently perform every task identified at that Red Hat URL, then I
strongly recommend that you take their class offerings. For me they were well worth it.
Just my 5 cents.