…those written in binary and — wait, that’s not right!

Actually, the two kinds of textbooks, at least in physics, are educational books and reference books.

• Educational books are good for learning a subject for the first time. They’re written in a way that makes it easy to read a whole section or chapter without getting overwhelmed. They have clear summaries, walk you through simple examples, and put their topic in context to keep you interested. These books are almost telling a coherent story as much as they are imparting information.

  Examples include David Griffiths’ books on quantum mechanics, electromagnetism, and particle physics, Daniel Schroeder’s thermodynamics book, James Hartle’s book on general relativity, and Barton Zwiebach’s book on string theory.

• Reference books are good for looking up the details of a specific procedure or situation. They’re densely packed with information, but that makes it very hard to read a large section at a time, the way you would read other books. Instead, a reference book is most useful when you need to know one particular fact and can look up just the part of the book that deals with that fact. In these books, the individual sections should be more self-contained because you don’t want to read through the whole book to get the information you need.

  Examples include Cohen-Tannoudji, Diu, and Laloë on quantum mechanics; Jackson on electromagnetism; Misner, Thorne, and Wheeler on general relativity; and Green, Schwartz, and Witten on superstring theory.

One of the biggest mistakes a textbook author can make is failing to decide which kind of book they want to write. While most textbooks have some features of each type, no single book can adequately fill both roles. Trying to do it just makes your book bad at both.

Of course, one of the biggest mistakes a textbook reader can make is expecting an educational book to be good as a reference, or vice-versa. A lot of trained physicists do this when they complain about Griffiths’ books being insufficiently rigorous, for example. Those books aren’t meant for trained physicists, they’re meant for undergraduates, who would just be confused by “sufficient” rigor.