Just a quick note to give you some feedback of my experience.
There are 3 different problems in the book that I found too hard. Even after reading all the hints - the penny still doesn't drop:
I spent roughly a week of my spare time to finish the book. A few were harder than others obviously, but the 3 above I believe I could have not completed these without the hints forum. I am talking about several hours of having all the clues essentially assembled and understood, multiple people studying the clues but we still can't get to the answer because of something being too obscure and/or non-intuitive.
Regardless, the books are amazing. I would buy another one too if it gets published.