I think readers are more likely to be annoyed by the reduced availability of used versions than directly by the reuse. That is, when a student sees that a copy of X for Engineers and of X for Trades are available but the student needs X for Artists and the books are by the same author and only differ modestly, the student might be somewhat upset. This is similar to the effect of frequently updating versions. This feeling would be amplified when the author is recognized as the teacher or being from the same department or university; the feeling of artificial profiteering is increased by the perceived lock-in.
Obviously the cost of the book in new and used forms, the availability of different versions (especially in used form), and the amount of shared material will influence the perception. If a new textbook is substantially less expensive than other textbooks for the field (e.g., art textbooks might typically be more expensive than textbooks in other fields), the annoyance of having to buy a more expensive new version is reduced.
If the different versions are released in different years, especially if the most structurally similar versions are the most separated in release time, then this feeling might be reduced somewhat. The perception is then conveyed that the more recent version is not just a rearrangement of content but an update. This release timing might also have practical benefits for the author and editors. Using more distinct titles would also reduce the appearance of reuse. E.g.:
- Practical X for Tradesmen
- Business Use of X
- An Engineer's Manual for [or Guide to] X
- X for the Creative Mind
If certain core material is both broadly applicable and enduring, it may be useful to provide a smaller number of versions of the core text (perhaps one for all users or perhaps two versions) with a supplemental paperback book with different versions. An ebook might be acceptable for a supplement which only included exercises and might reduce the cost of diversity. While having to buy (and use) two books would probably annoy students, the annoyance of having to buy a $35 used version of the core and a $20 new version of the supplement might be less than that of having to buy one $80 book when a $40 used version with mostly the same material is available.
In general people do not have an intuitive sense of the costs of developing and making available a textbook. If a textbook has unused chapters, a person is likely to feel that the book would have been cheaper if the excess material was excluded, not realizing the interplay between fixed and incremental costs. If a book has 80% of the same content as a book with half the price (even when comparing new to used), there may be an impression of being cheated.