I feel Jira Admin exam is not only about "how much you know Jira admin" but rather "the best way of doing Jira admin in different situations". I don't think the existing books (including mine) were written with a purpose of providing a guide to clear the exam. We are anyways not allowed to share sample questions asked :)
Each book is based on experiences of author when they were working as Jira Administrators. You will learn something different from each book. Comparing them is probably not a good idea.
Someone who is already a Jira admin and wants to clear the exam. Here are my tips.
1. Some preparation before you start a proper preparation
Do sample exam, it is short but will help in assessing where you are.
Make sure you already know Jira customisations, if not delay your exam.
2. Based on features of the tool (You don't want to loose marks just because you forgot something).
Get a test instance of Jira Server. No add-ons.
Download the exam topics and print them.
Open Atlassian documentation and read those topics word by word (EXTREMELY IMPORTANT). Everything asked will be somewhere in the documentation.
While you are reading those topics, practice on Jira instance (VERY IMPORTANT). You want to remember different things on the UI. The questions will try to confuse you but if you recently did a hands on then you will remember what you practiced.
Make plenty of notes on paper for each topics (VERY IMPORTANT). This is just to fill your brain with terms, keywords, possible options that appear on the UI. This worked for me, If I write something, most likely I remember it for the next few days.
Before the exam, I prefer reading my notes.
I prefer going to the centre hour in advance, nervousness of the exam, going to the centre helps in keeping my focus.
3. Based on your experience
If you have been working as a Jira admin then you probably know when not to create another set of configurations or better ways of doing things to save time and optimising existing configurations.
Just use your best judgement
4. For exam
Round 1: Spend one hour answering simple questions, that requires no further thinking. That you can answer straightaway. Less than 1-2 mins per question max.
Round 2: Answer questions that requires some thinking, in those cases reading answers will give you some clue.
Round 3: Review
Answer everything (last time I checked there was no negative marking)
Bonus tips: Don't drink too much water before/during the exam ;), wear relaxed clothes and be your best to feel confident.