Writing at The SF Site, Regina Lynn has an open letter to publishers asking that they include a volume number on a book’s cover if the book is part of a series so that a reader can determine the order of books in the series without even needing to read the book’s jacket blurb and potentially learn spoilers about the plot of earlier books in the series.
I think this would be very useful when possible, through in some cases there is not an established order. Determining the reading order for books in the Vorkosigan saga by Lois McMaster Bujold is the subject of many an argument – by publication date, chronological, or more important or better written books first?
One thing that is definitely needed is an indication if a book is part of a series, trilogy or other multi-part volume, I was wondering why nothing much was happening in Harry Turtledove fantasy novel Beyond the Gap when it got to the last few chapters and I realized that it was the first book in a series, a fact not at all mentioned on the cover or front flap.