When I buy individual books I’m not picky about what format they are. Hardcover, paperback, mass market, it doesn’t really matter, especially if I find them secondhand or at a library sale. I buy whatever format I find. Unless I already own a book from a series and I’m buying another book from the same series. In that case they have to match or I probably won’t buy the second one.
Although my shelves house a variety of sizes, styles, and backings, all of the series on my shelves are matching. Whichever format I started buying a series in, I’ll make sure I continue to buy that series in that format. It doesn’t bother me that other people don’t care if their books match, but there’s only so much chaos I can allow on my own shelves.
Sometimes it’s impossible to do this though. Sometimes an author changes publishers during a series or publishers re-design a book before the series is finished. In either case, the first half of a series may be styled one way while the second half is completely different. Your completed series will never match unless you re-purchase the ones you already own. Even if the new designs are better, that’s annoying. Even worse if the old designs were better.
This is really a silly aesthetic preference. The story inside the books are generally the same from one format to the next. Books go out of print and libraries don’t have everything ever published. I’d rather have a mismatched series that I got to enjoy than stop reading something I love because my covers wouldn’t match. Still, I’ll try my best to make sure they do match.
Do you mind when a series changes design before all the books are published?