I want to hire a graphic designer to design one cover to be used on a series of books. Each cover in the book will have the same design, but slight modifications to each. For instance, the first book might say “Book 1”, and have a red tint, while the second book might say “Book 2” and have an orange tint, while both books have the same pattern across the front and back.
Would it be acceptable to ask the designer to just make a single design and to leave me with an editable file, where I can manually change the text and tint myself each time I am ready with a new book for the series? Or do I need to contact them again each time I have a new book in the series ready and ask them to do this?
As long as this is a clearly understood arrangement from the start from both parts, you certainly can. Be upfront about it, write a clear contract, and all is well.
No designer would be offended/annoyed as long as it is abundantly clear from the start.
As @Joojaa points out, a very important thing: make sure you agree on the software beforehand, so that you actually have the ability to alter as you please. In addition, also agree on fonts that you have available, or fonts that you are willing to pay for.