The depth and breadth of topics and examples is amazing. Well done! My only nitpick would be the formatting of some of the equations. Instead of $$ E = hf

The depth and breadth of topics and examples is amazing. Well done! My only nitpick would be the formatting of some of the equations. Instead of $$ E = hf $$ $$ E = (6.626 \times 10^{-34})(10^{9}) $$ $$ E = 6.626 \times 10^{-25} \, \mathrm{J}$$ I think it would be clearer to use $$\begin{aligned} E &= hf \\ &= (6.626 \times 10^{-34})(10^{9}) \\ &= 6.626 \times 10^{-25} \, \mathrm{J} \end{aligned}$$ which is the KaTeX-recommended approach for emulating \begin{align} ... \end{align}. I would also throw in a word or two of explanations in there, so it's not just equations, e.g., explain "Using the formula for the energy of a photon, we find: <eqns>". I guess it would end up being more work for you, but would be worth it for the benefit of students learning that it's not just about the numbers, but knowing what you're doing (like you do in the strategy sections). On the other hand the current format of line-by-line-show-only-the-equation-steps is very practical too --- from my experience certain students prefer to see just the practical steps they have to do on the exam, rather than the blah blha blah around them, even if it can be educational. It would be interesting to see some of the questions/examples converted into self-test quizzes. Show a few examples, then ask readers to solve some problems on their own. I know the idea is they will try to solve on their own an only reveal the solution after they've tried, but realistically speaking more likely they will click that arrow ;) So maybe just a delay to force them to try? This is getting into the realm of the fancy, but you could maybe make the reading with a "customizable difficulty level." Beginner = info (maybe hides certain more advanced topics), all questions solved as examples. Intermediate = some questions solved as examples, but missing final steps so reader has to complete them to get the final answer. Advanced = most examples show as quiz questions reader has to solve on their own...