Here are some examples of really accessible textbooks:
- DINOSAURS and Other Prehistoric Reptiles. A Giant Golden Book.
- How to Lie with Statistics. By Darrell Huff; illustrated by Irving Geis.
- How to Win Friends and Influence People. By Dale Carnegie. 1937 edition.
In my opinion, the best textbooks take an approach of:
Limiting the scope to things that a non-expert reader is likely to find interesting.
Consciously treating the field as containing mysteries:
- Asking "How can we figure something like this out?"
- Showing that even experts make mistakes, and that ordinary people can figure things out.
- Providing examples of the limits of what is known at the time.
Illustrate the ideas with pictures and/or examples.
Show a clear, practical, step-by-step way to solve each solvable problem.
Show a clear, practical way to check how accurate (or useful) each answer is.
These books tend to be well-organized, with big fonts and clear summaries. They emphasize "you can do it yourself" sanity checks rather than proofs.