I'd like to make two recommendations to add to the list.
1) Under the "Everyday basics" category, a high school intro chemistry textbook is a must, especially if people want to understand what pH is, how bicarbonate works as a buffer, why 5.0ppm ammonia at pH 6.5 and 20C (68F) isn't as harmful to fish as 0.5ppm ammonia at pH 7.8 and 26C (79F) is, the difference between organic and inorganic phosphate and why one causes algae blooms and the other doesn't, etc...
2) For the "Advanced aquarist" a gram scale, for properly measuring out the correct masses of salt, powdered medicines and fertilizers to make achieving the correct concentration of a solution easier and more accurate, especially when dealing with things like copper sulfate...