I'm thinking about textbooks this morning because - well, I'm a teacher who uses textbooks. Not all teachers do - my graduate school professor of medical entomology scorned textbooks and insisted we use the original literature. Each class began with a long list of references which applied to the topic of the day. That is, perhaps, the ideal way to teach, but it's impractical in public schools and in places like my college, where I teach so many courses. I'd lose my mind trying to keep up (actually, I'm pretty close to losing my mind even with the "short cut" of textbooks)!
However, the approach my grad school prof took avoids the main flaws of textbooks, which include being out of date the moment they're published and enshrining incorrect information in the minds of students. Because, let's face it, a "textbook example" is not all it's cracked up to be.
Right now, for example, there's a "debate" raging on a message board I follow about Haeckel's fraudulent drawings of vertebrate embryos. Those drawings (or rather copies of them) made regular appearances in biology textbooks until the relatively recent past. The point the textbook authors (and Haeckel) were trying to make was that the similarity of vertebrate embryos at early stages of development indicates an evolutionary relationship between broad classes of vertebrates (fish, reptiles - including birds - mammals). Haeckel, either consciously (=fraud) or unconsciously (=confirmation bias) over-emphasized that resemblance in his drawings. Creationists have a great time with this "fraud", all the while ignoring the fact that Haeckel was correct in his basic point: the resemblance of vertebrate embryos at early stages of development does indicate an evolutionary relationship. In any case, the continued use of these (copies of) drawings after the exaggerated nature of the drawings was known is but one example of "enshrining incorrect information". These days, most textbooks use actual photographs of the embryos - and guess what? - they show a close resemblance between broad classes of vertebrates indicating an evolutionary relationship between them.
Which begs the question - why did the authors of textbooks continue to use the (copies of) drawings long after their true nature was known? I think there are several reasons. One would be inertia and its cousin - laziness. Once one text author used them, and they seemed effective, all the other text authors would use them too. But effective at what? Well, in one picture students could see how closely related diverse groups of vertebrates are - a picture is worth a thousand words. It was a "good" teaching tool - and that's what textbooks are all about. And that's the second reason - text authors are most interested in teaching, and anything that helps get the point across in a simple way seems like a good idea. Text authors are faced with a big dilemma - how to get complex information across to students in as simple a way as possible? The fact that Haeckel over-emphasized what turns out to be a true phenomenon (as true as anything in science) seems like a minor detail that can be ignored in the face of what the author is trying to convey.
Let me give one more example of "enshrined information" that I think is more problematic: the treatment of Jean Baptiste de Lamarck. When I went through school, Lamarck was always taught at the guy who got it wrong. He's the guy who believed in "use and disuse" and "inheritance of acquired characteristics" and - ha, ha - what a fool! Darwin, on the other hand, didn't believe such nonsense and so we can see how the neck of the giraffe evolved according to Darwinian principles. And then I read On the Origin of Species by Means of Natural Selection ... and - surprise, surprise - use and disuse and inheritance of acquired characteristics were all over that book. I'd been lied to!! Well, the textbook authors didn't intend to lie - they were just looking for a simple way to differentiate between Lamarck and Darwin and hit on those two things because it would be "easy" for students to see the difference. And I suspect many of the text authors didn't even know that what they were presenting was flat-out wrong. After all, they'd been taught the same thing and probably had never read On the Origin ... (many biologists never do!).
It turns out that there are some major differences between Lamarck and Darwin, but they don't include differences in their knowledge (or lack of it) of heredity. Textbooks are ever so slowly coming around to facing the complex nature of the differences between Lamarckian evolution and Darwinian evolution, and that's a good thing. But you can bet your textbook has other wrong information buried in it - not out of bad intent, but simply because the authors are trying to find the balance between presenting complexity in all its twists and turns, and conveying important information in as simple a way as possible.