Editing any book on any subject involves the four C’s: clarity, consistency, choreography, and comprehension.
Toward this end, professional editors a create style sheet to manage all the details; show the author what the editor has done on a macro level (vs. the micro level of in-manuscript changes); and give any future editors and proofreaders a resource to help with their own work.
This document also provides a foundation for both author and editor to build upon in crafting series fiction.
Few editors can retain details from volume 1 and recall them instantly when faced with the next volume(s) months or years later. This makes style sheets crucial for reference—doubly so if a different person edits each book.
Style sheets establish not only technical conventions to apply to the story (spelling, capitalization, italics, proper nouns, unique phrases), but also capture character traits and relationships, places, peculiarities, and timelines. These may come into play in later episodes but be forgotten over the time it takes to compose the next book.
Authors who write their first book knowing another will follow often have their own notes to aid in continuity. Sharing these up front helps orient the editor, and saves time and therefore cost. But sometimes authors get surprised—what they expected to be a single book blossoms into a sequel(s). So having a style sheet from the first book helps the author forge ahead on the next, free to concentrate on story.
It serves an author and editor equally to do a full series together. The editor will have book 1 embedded in their consciousness, including decisions made and problems solved while compiling the initial style sheet, whereas changing editors forces the new person to start from scratch. This can be good if a fresh viewpoint is needed; not always good, in that new influence can knock the author off stride, and certain baseline work will have to be redone.
Chemistry and schedule, of course, are the determining factors. What matters is that a comprehensive style sheet be developed for each book, expanding on the one before, just like the story develops through the series.