Now that there's no longer a 20 wpm code requirement for Extra, there's very little reason why almost any ham with some good basic math skills can't upgrade, given a book this good. All it takes is reading the book and flipping to the questions to check your understanding of the material as you finish each section. I found it took me about a week to do that and I passed with just one mistake (which kills me because if I'd taken more time, I should have gotten that one, too!)
I found the book quite logical, well-written and very methodical at explaining the material. I only spotted two minor errors: A garbled explanation of DeMorgan's Theorem as "logic polarity" on page 7-5, and the schematics of three oscillators on page 7-42, each with two connections to Vcc but none to ground.
The whole point of the book is to help you pass a multiple choice exam where all the questions are taken from a fixed pool and everyone knows what they are. But the book nicely avoids the trap of simply helping you memorize the answers and focuses instead on explaining the concepts so that when you read a test question, you're capable of doing whatever calculations are required and you know why one specific answer is the best. The only places where I felt the author might have offered a little more discussion were in presenting the formulas for second and third-order intercept points on page 4-22 (it took me several hours to figure out where these came from!) and in the section on phased vertical antennas (page 9-20), where you're given 15 charts to memorize but little explanation of how you might intuitively figure out what shape to expect as an alternative to the rote memorization.
I recommend this book. Go get your Extra!