I often wonder why libraries will have completely arbitrary books from a series. In this case the library only has books 9, 10, and 16. Maybe those are just the ones people have specifically requested for the library to purchase? If so, why did they request only those? This happens a lot both with ebooks and physical ones and has always made me curious. (This isn't a gripe, just an idle thought. I know I can contact them to request more.)
@kristina Possibly the rest were lost and/or weeded due to poor condition, nobody got around to checking they still had a full series, or the missing titles were out of print by that point and couldn’t be replaced, or the vendor/platform only had those titles, or maybe someone really only requested books 9, 10 and 16… all kinds of reasons. That kind of proactive collection development in public libraries is pretty rare these days

@kristina We always bought complete series but I guess some might do this when, as you say, specific ones are requested because the user doesn’t have that exact volume and so asks …

Or, the librarian on duty for buying books has no good idea about the series but thinks the library should have it. Then the next time it’s someone else without any idea about the series and skips the purchase?