In the book, he thoroughly covers what is far and away the most commonly used method of synthesis in electronic music. It goes through all the fundamentals—oscillators, filters, envelopes, LFOs and so on. It presents a wide range of modulation routings, including many you might not have thought of. There's an introduction to FM, AM, ring modulation and additive synthesis. He also provides plenty of examples of how to program staple sounds and achieve a certain character in a sound.
All the relevant information is there, but the real gold is the understanding that Papen lends to the topic. He not only explains fundamental concepts like frequency and phase but also shows how they apply when programming synths. One memorable example is his explanation of why we sometimes get unstable fundamental frequencies in our bass sounds and how to avoid them. He gives tips on building a sound from scratch and explains how to reverse-engineer your synthesizer's presets to better understand why they sound the way they do. He also highlights parallels between synthesizers and acoustic instruments, with an emphasis on what the latter can teach us about the former.
His teaching method is very clear throughout. Learning about subtractive synthesis by focusing on Papen's four elements—oscillators, filter, amp and modulator—resembles learning from a modular synth. This is generally acknowledged as an excellent (if prohibitively expensive) way to learn, as a lot of the architecture of a typical synth is invisible. Papen's method gets you in touch with that architecture without the massive investment. He takes things at a beginner's pace and goes over some of the more complicated ideas a number of times. Rather than a book of condensed information, it reads as if you're being taught directly, and on the videos he speaks to you as if you are, too.
Throughout all this, he also instills some important philosophies. You may be familiar with the idea that less can often be more. But it might be something of an eye opener to really understand that, for example, just one oscillator can be all that's needed to get a fat and interesting bass sound. He often points out that what you're really after is the right sound for your track or for your particular style. And he always encourages you to experiment and think outside the box.
Presumably, if you're interested in learning subtractive synthesis, you also have at least a passing interest in synths themselves. In the video lessons, he uses a range of lovely synthesisers, particularly the MiniMoog, the Korg MS-20, the Roland Jupiter-8 and a Synthesizers.com modular. This range gives a good sense of the differences in functionality between synths, and he also provides good insight into their historical significance.
Whether the asking price, which is comparatively high for a book, is justified for you depends on what you want to get out of it. If you're an intermediate producer, there are likely other resources that can give you what you want at a cheaper price. This is more of a course than just a book, though, and it's an excellent choice for anyone looking to deepen their understanding of synthesis. If you're just starting out and want to gain a very firm foundation in this subject, I can unequivocally recommend Papen's method.