It is not a math book rather more of a puzzle book. On a side note - it's full of mistakes (your experience may vary though)
If you like to see difficult math then please try to apply for a self-driving car engineer's job.
Pretty sure the prize for 'most difficult math book in history' goes to Dummit and Foote's ponderous tome, Abstract Algebra.
Besides, cracking the code interview is definitely the hardest. Just check out the amazon reviews