I really enjoy reading the book. The most useful part, in my opinion is the solutions. I can solve some of the problems, but after reading the solutions, I learned new design/ideas and implementation skills. I really enjoy reading the solution for each problem.
Another interesting thing is the variants to each problem, which are both interesting and valuable. Unfortunately, there is no solution to invariant. I would be really appreciate if you share out the solutions for variants on the website. I am even willing to pay a separate price for the variant solutions! At least, I would hope you can post the code for the variants(since your code is quite readable and meaningful) for learning purpose.
Thanks again for writing such a wonderful book and have a good holiday weekend!