This is what I wrote to Dr. Borovik about this book.
Dear Professor Borovik,
I can't thank you (and Dr. Tony Gardiner) enough for writing Essence of Mathematics and making it available for free download at the Open Book Publishers' page. I am reading (and independently solving all the problems from) it along with my son (a high schooler). It is a joy to read such books. I wanted to express my deep gratitude to both of you.
Best Regards,
Kedar