Textbooks, especially math and science, should have the answers to all the problems given in the book (not just the odd ones, etc.) located at the end of each chapter rather than at the end of the book.
Books, esp. educational textbooks, should have phonetic pronunciation guides for either all fairly complex or rare words and/or for all words that may be sometimes mispronounced by people or which are significantly more likely than average to be mispronounced. Preferably, these phonetic pronunciation guides should be located right next to the word either in the text, in the margin, or at least on the same page of where the word first appears. In addition, and at the very least they should be located at the end of the chapter or book, such as in a glossary of terms section.
Glossary of Terms
Every textbook should have a glossary of terms.
All textbooks (especially math books) should explain and show, in detail, every single step of problem solving without skipping or combining steps and without assuming that the student would know to accomplish a step without being shown, regardless of how simple or seemingly insignificant the step appears to be.
All textbooks, especially 10th grade and higher, should contain an exhaustive index.