I agree that it's too long, but most of what's said could be retained if only it was popup text that came up from links, or page subsections linked from an outline at the top. Your section titles would be the link titles, basically. Then readers can quickly zero in on the section that answers their questions. If you then had the buy button visible at all times, such as in a sidebar or as a separate link in the link "menu", I could find my answer quick and buy it now.
Not a designer here, but thinking like a customer.
By the way, I bet you could sell more $ if you had three purchase levels instead of two, supposing that you want to put in the work to create a third level of the product. The third level would be more expensive. Its biggest purpose would be to "frame" the Pro book so that $100 feels inexpensive.
I am right at the edge of being an actual customer because once or twice a year I flirt with exiting FIRE and getting SQL-ish job. This is a good product for me. Going in, I value the practice and recognize the need to re-sharpen my skills. I'd be insecure and lazy, though, so I'd LOVE a subscription service - say, the 57 problems, plus some sort of additional assistance or offering that's not in the book, in exchange for say $19.99/month (9.99/month if you sign a 1 year contract that automatically renews). Additional assistance could include or consist of answering subscriber questions about SQL, and access to the questions/answers from other subscribers. If you want to add more practice questions, something that returns measurement data in addition to right/wrong seems like it would be really cool (time spent on problem vs average time, time distribution that others spent on the problem, etc. ... helps enliven the lonely practice experience, at least in my imagination.) Call it the SQL ACES subscription. Making stuff up here, ACES could be Access Club of Excellence in SQL.
Do I get a free subscription for my fabulous ideas? I mean, inventive beta testing invitation? :)