That's partly because people write for several audiences simultaneously. Once you say, "Oh, but some of my readers won't know X, I can't assume that..." It's an

That's partly because people write for several audiences simultaneously. Once you say, "Oh, but some of my readers won't know X, I can't assume that..." It's an endless question, so at some point you have to admit that you can't write the most accessible book in the universe. But if you begin to even think "Oh, but some of my readers won't understand X" and you write a little supplemental chapter or section, the book starts to engorge. Then you go to one of those condensed books for math and you think about what kinds of people meaningfully consume these books.