Author here. Thanks for the heads up, it did look a bit bleak on the mobile. Increased the font size, made grey much more dark, and increased heading sizes. Hope it's easier to read now!
Advice to all web designers: Please do not use "font-weight: 300" for body text, ever. Simply changing that to 400 in the developer tools made it much more readable. The font was still bugging me, so I tried killing the "font-family: futura pt" too.
Wow! What a world of difference. Now it is super easy to read, instead of looking like it's trying to show off some design style.
Know your audience: you are addressing potential database developers with this book, not graphic designers.
I also want to apologize for my comment being a little grouchy; I should know better than to comment on style and readability in the wee hours. :-)