I wouldn't call it hard, but one should read Discover Canada in its entirety.
The way I did it.. I read the whole book once (over a period of 5-6 days), highlighted things I considered important and then re-read that part (within a day), took the exam after second reading. I was able to score 20.
@sundeep_s_ shared a post sometime back with a link to different websites:
https://www.canadavisa.com/canada-immigration-discussion-board/threads/august-2021-citizenship-applicants.740188/page-87#post-10008132
I only used hicanadatest and yourlibrary.ca to do some practice questions but apart from giving me the confidence (which was definitely helpful), I don't remember seeing a lot of questions from those tests though. I didn't pay a dime for any of the practice material, I don't think it's really required.
In the end I would just say read the book, be calm and just go for it!