I passed the FCNSA last week with a score of 97%. It was a lot more easy as expiated compared to Cisco exams.
There is no good book or something like that available, but for this exam there is not use for in my point of view. It' s enough if you have hands on experience.
Spent some time with the following features, and you will pass. You have to reach 60% which is not very challenging.
Look on the following topics.
Firewall rules in general, also Identity based
UTM features and assignment
What is possible with which UTM feature (and what is not)
FortiGuard status within GUI
redundant internet uplinks
Site2Site VPN configuration
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