1. It should be open-source or available on your nearest "App Store" for a REASONABLE price
2. It should be interactive. If I have a history textbook describing a battle, I want to be able to watch the battle unfold (even if it's just a diagram). If I am reading a chemistry textbook, let me play with the molecule. When I need to learn about the D3 group in Abstract Algebra, show me a triangle rotating and flipping as the group operations fade in and then drop to the bottom in a chain illustrating a composition. Let me swipe over to the Cayley table and touch a composition cell and watch the triangle dance. Whatever.
3. Textbooks should share a common ontology that can be extended for each book. But if I have two textbooks on my device and each has registered its ontology, I should then be able to cross reference concepts from one book to the other, or to appropriate entries in Wikipedia, for example.
4. All Chapters, headings, subheadings, and definitions should link to global, school, and class-level discussion forums about the topic where everyone can ask questions and have good answers upvoted in a stackexchange-like manner. These discussion nodes should be linked to the ontology, rather than the brand of book. That way ANY textbook with a section on "Limit Points in Rn" or "The Three Branches of US Government" would link to the same discussion node.
5. When appropriate, texts should be constantly updated, classes should always use the "latest stable version", and that update should be free after initial purchase (like an App).